Refactor permission checks with issues and merge requests project settings

Slight refinement for read/write permission checks for 04d44522 as discussed in !691. Currently it appears that there is [no role that only has read-only access to issues](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/permissions/permissions.md), but it could be possible.

/cc: @DouweM

See merge request !704

Remove Rack Attack monkey patches and bump to version 4.3.0

I finally got these monkey patches into Rack Attack v4.3.0, so GitLab no longer needs them. Hooray!

See: https://github.com/kickstarter/rack-attack/pull/128

See merge request !693

Added documentation styleguide

Hi. I added a documentation styleguide and linked it to https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides

See merge request !709

Added note about backups for gitlab.com

Added note about backups for Gitlab.com based on question on Twitter
https://twitter.com/MarcelloLins/status/598544357294272513

See merge request !1828

Document expanded autolinking in doc/markdown/markdown.md

See merge request !706

Time zone text fix

Added small fix to /etc/gitlab/gitlab.rb

See merge request !1830

Disable "New Issue" and "New Merge Request" buttons when features are disabled in project settings

Closes #1676

See merge request !691

Closes #1676

Remove redundant CHANGELOG entry

This was moved into v7.10.4, but I forgot to delete it from the v7.11 list.

See merge request !692

Add support for Webhook note events

This MR adds support for note events (comments) for WebHooks.

Perhaps the only potential source of confusion is that the settings indicates "Note Events" instead of "Comments". What do you think?

Screenshot after:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/299b141d82f7b14943a74258d5c57ca6/image.png)

Closes https://github.com/gitlabhq/gitlabhq/issues/6745

See merge request !673

Keyboard shortcuts

Added doc about how to view keyboard shortcuts

cc/ @sytse

See merge request !1829

Use the user list from the target project in a merge request

### What does this MR do?

This MR derives the assignable users from the target project.

### Why was this MR needed?

In a merge request, the assignable users uses the list from the forked project, not the target project where the merge request will be going. For example, if I fork a project from the gitlab-org namespace into my personal namespace, make some changes, and then create a merge request, the only users I see are me. I think it makes more sense to show all the users in gitlab-org. This is a regression caused by 1c2711f7.

### What are the relevant issue numbers?

Closes #1535

See merge request !682

Closes #1535