Add negative tabindex to Markdown Write and Preview tabs

Closes #1757

See merge request !796

[Docker][Documentation] Uses correct docker image from Docker Hub

[Docker][Documentation] Uses new docker data image from Docker Hub (*sytse/gitlab-data*). Uniform tag name for data image is now *gitlab-data*.

Info about namespaces

Added info about namespaces in groups.

Because of tweet https://twitter.com/kaystrobach/status/603912904757420032

See merge request !1835

Update grids for 2FA details

Also adds a link to the documentation.

Closes #1758

See merge request !795

fix and minify images

See merge request !1853

move and fix links importing docs

See merge request !1852

Update oauth button logos for Twitter and Google

Both new buttons are 64x64 renders of the recommended assets
from each company.

Refs for those recommendations are:

 - https://about.twitter.com/press/twitter-brand-policy
 - https://developers.google.com/identity/branding-guidelines

Closes issue #544

[ci skip]

Sign in UI before:

![sign-in-buttons-before](https://gitlab.com/fmborghino/gitlab-ce/uploads/ed42a056b23eb6db3907cb604a865ce5/sign-in-buttons-before.png)

Sign in UI after:

![sign-in-buttons-after](https://gitlab.com/fmborghino/gitlab-ce/uploads/c7fb1d996fa744274298e195799bc033/sign-in-buttons-after.png)

See merge request !798

Both new buttons are 64x64 renders of the recommended assets
from each company.

Refs for those are:

 - https://about.twitter.com/press/twitter-brand-policy
 - https://developers.google.com/identity/branding-guidelines

Addresses issue #544

Fix alignment of Project Settings nav icons.

@douwe thanks for reviewing this small fix.

![before_after](https://dev.gitlab.org/gitlab/gitlabhq/uploads/ce213aa5b08c640ec5e9b018e0ec4036/before_after.png)

See merge request !1848

[ci skip]

Closes #1757

Also adds a link to the documentation.

Improve project edit form

Properly align checkboxes according to bootstrap

Before:

![Screenshot_2015-06-10_17.01.26](https://gitlab.com/gitlab-org/gitlab-ce/uploads/2e874cc0a6edd52d98a17030f3d16f24/Screenshot_2015-06-10_17.01.26.png)

After:

![Screenshot_2015-06-10_16.59.31](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7d91ffe8e981f9bc57d9a18ce8ac79e2/Screenshot_2015-06-10_16.59.31.png)
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !793

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Changes back-ported from rspec3 branch

This is just breaking up the large `road-to-rspec-3` branch a bit. Each of these commits have been cherry-picked and would be good to have in place before the upgrade.

See merge request !792

update fog to 1.25.0 and add multipart upload support

This merge will fix backup uploading problems to s3, closing gitlab-org/gitlab-ce#1059

See merge request !789

fixed links

@job I am trying to fix these links, since they work on the commit, but then they didn't work on the website when the commit was merged.

I tried changing them here, but I'm not sure if they'll work when merged. I tried to use localhost to test this, but it only works with www-gitlab-com

Thanks!

See merge request !1849

Update browser gem to version 0.8.0

Internet Explorer 11.0.9600.17633 has a new User-Agent string (tested using [BrowserStack](https://www.browserstack.com)):

```
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
```

This is supported in v0.8.0 of the browser gem:

https://github.com/fnando/browser/commit/aab7a81d94cc2c31c4b860fa4a9b9a361adf1bab

Closes #1775

See merge request !791

They're called constants for a reason, after all.

This might see a minor speedup in test cases that call this method many
times.

As of Ruby 1.9, this is the correct way to handle `respond_to?` for
methods implemented by `method_missing`.

See https://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding

Closes #1775