Makes reCAPTCHA configurable through Application Settings screen

Following the work made by @stanhu here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2216, made it configurable without needing to restart Gitlab

See merge request !2231

Fix spelling mistake, thanks Connor.

[ci skip]

See merge request !2227

Handle missing settings table for metrics



See merge request !2232

Fix broken link in permissions page



See merge request !2233

This ensures we can still boot, even when the "application_settings"
table doesn't exist.

Move InfluxDB settings to ApplicationSetting

This moves the settings from the YAML files to the database.

cc @sytses

See merge request !2228

Fixed syntax in gitlab.yml.example



See merge request !2230

Restart settings are moved too.



See merge request !2226

add issue weight to contributing

cc @dzaporozhets 

See merge request !2223

Storing of application metrics in InfluxDB

This adds support for tracking metrics in InfluxDB, which in turn can be visualized using Grafana. For more information see #2936.

See merge request !2042

Environment variables in the app

Fixes #3717 and #3519 

## Why environment variables?

We need environmental variables, they are an expected way to configure apps https://medium.com/@kelseyhightower/12-fractured-apps-1080c73d481c#.ntrdiyu4c

This causes many tools and to tutorials to make it easy to set environmental variables and harder to supply a configuration file. 

So even though we agree they are not ideal https://support.cloud.engineyard.com/hc/en-us/articles/205407508-Environment-Variables-and-Why-You-Shouldn-t-Use-Them the market has spoken.

## Why for GitLab the application and not for the Omnibus packages?

Environmental variables are also needed by people that do not run our Omnibus packages, for example natively bundled apps (Debian apt-get) and idiomatic Docker packages (Mesos, Kubernetes, etc.).

Of course it should work great with Omnibus packages too so any advise is welcome in that regard.

There is an MR https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/575/diffs to be able to set any variable in gitlab.rb via environmental variables. I think both that and this MR should be merged to solve the configuration problem for both Omnibus and non-Omnibus installations. When both are merged the documentation should be crosslinked.

## Why uppercase?

Need to be all cap according to Google Shell guideline:
"Constants and Environment Variable Names => All caps, separated with underscores, declared at the top of the file."
https://google.github.io/styleguide/shell.xml#Constants_and_Environment_Variable_Names

Or as explained on http://stackoverflow.com/a/673940/613240
Keeping to this convention, you can rest assured that you don't need to know every environment variable used by UNIX tools or shells in order to avoid overwriting them. If it's your variable, lowercase it. If you export it, uppercase it.

/cc @JobV @DouweM @marin @jacobvosmaer @ayufan @pravi

See merge request !2215

Disable --follow in `git log` to avoid loading duplicate commit data in infinite scroll

`git` doesn't work properly when `--follow` and `--skip` are specified together. We could even be **omitting commits in the Web log** as a result.

Here are the gory details. Let's say you ran:

```
git log -n=5 --skip=2 README
```

This is the working case since it omits `--follow`. This is what happens:

1. `git` starts at `HEAD` and traverses down the tree until it finds the top-most commit relevant to README.
2. Once this is found, this commit is returned via `get_revision_1()`.
3. If the `skip_count` is positive, decrement and repeat step 2. Otherwise go onto step 4.
4. `show_log()` gets called with that commit.
5. Repeat step 1 until we have all five entries.

That's exactly what we want. What happens when you use `--follow`? You have to understand how step 1 is performed:

* When you specify a pathspec on the command-line (e.g. README), a flag `prune` [gets set here](https://github.com/git/git/blob/master/revision.c#L2351).
* If the `prune` flag is active, `get_commit_action()` determines whether the commit should be [scanned for matching paths](https://github.com/git/git/blob/master/revision.c#L2989).
* In the case of `--follow`, however, `prune` is [disabled here](https://github.com/git/git/blob/master/revision.c#L2350).
* As a result, a commit is never scanned for matching paths and therefore never pruned. `HEAD` will always get returned as the first commit, even if it's not relevant to the README.
* Making matters worse, the `--skip` in the example above would actually skip a every other entry after `HEAD` N times. If README were changed in these skipped commits, we would actually miss information!

Since git uses a matching algorithm to determine whether a file was renamed, I
believe `git` needs to generate a diff of each commit to do this and traverse
each commit one-by-one to do this. I think that's the rationale for disabling
the `prune` functionality since you can't just do a simple string comparison.

Closes #4181, #4229, #3574, #2410

See merge request !2210

Fix the "Show all" link for the keyboard shortcut modal

18cb430f introduced a typo that made this stop working.

See merge request !2218

Add support for Google reCAPTCHA in user registration to prevent spammers

To do:

- [x] Failing reCAPTCHA test causes all the fields to be lost
- ~~[ ] Improve styling of reCAPTCHA box~~ (not possible)
- ~~[ ] Put settings in `application_settings` (?)~~

![image](/uploads/d38ca89820d3c0066fb8aeb645fd77f0/image.png)

![image](/uploads/6b050749963691b023d076682abcf736/image.png)

Page when you fail CAPTCHA:

![image](/uploads/bc4846f0a5144985bc41dfa75eeab4c1/image.png)


See merge request !2216

Bump brakeman to ~> 3.1.0



See merge request !2219

update-init-script was listed two times. removed one without explanation.

`update-init-script` was listed two times. removed one without explanation.

See merge request !2170

[emoji-picker] Do not show frequently used category when it's empty

I added one condition.

See merge request !2221

Fix intermittent timeout problems with feature specs.

This MR updates the gitlab-ci config to compile assets before running feature specs.

Closes #4662

See merge request !2220

Only allow group/project members to mention `@all`

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3473

See merge request !2205