API: List issues for a particular milestone

# What does this MR do?
This MR adds a new API method for retrieving the issues associated with a particular milestone. The resource is described at ":id/milestones/:milestone_id/issues", returning a simple array of issues.

I've picked up gitlab-org/gitlab-ce!113  and fixed the test for it.
From the original MR:
This feature was motivated by an in-house project for reporting--for large projects, retrieving and filtering the entire issue list by milestone was becoming too slow, particularly when pagination required multiple requests. While this resource also uses pagination, it allows the consumer to limit reporting to the particular milestone they are interested in.

@dblessing Please take a look :)

See merge request !308

Conflicts:
	CHANGELOG

Upgrade to Sidekiq 3.3

Fixes #1807.  Upgrades Sidekiq to the latest stable release.

cc @sytse

See merge request !1419

The previous recommend incantation would leave the process we read from
hanging around, even though it had finished. That gives you a
'defunct'/'zombie' process.

Append in place for strings and arrays [failure unrelated]

Validation of 'Create branch' and 'Create tag' buttons

Fix various typos

Conflicts:
	CHANGELOG

Disable 'check all issues' checkbox for unprivileged users.

Fix spinner icon to match others

Fix sentence in notification page

Correct the sentence in profile notification settings page.

/cc @jacobvosmaer @dzaporozhets

See merge request !1421

Remove unnecessary / from avatar url

## What does this MR do?

This MR removes the unecessary / from the avatar url.

## What Use Case does this MR solve?

The // in the url cold lead to avatars not being displayed (reported on IRC)

With this MR the avatar url changes from
http://localhost:3000//uploads/user/avatar/1/gitlab_logo.png
to
http://localhost:3000/uploads/user/avatar/1/gitlab_logo.png

@dblessing Can you take a look?

See merge request !303

show diff in submodules

Allow commit messages to close several issues at once

Show tags in commit view

### What does this MR do?
With this MR you can see the assigned tags for a commit in the commit view.
### Are there points in the code the reviewer needs to double check?
I don't think so. But it would be awesome if the reviewer can tell me how to add a tag to the sample_commit in spec/support/repo_helpers.rb.
Then I can add a test for the new tag_names_contains method as soon as possible :)

### Why was this MR needed?
At the moment if one click on a commit will see details such as: parent commit, amount of additions, deletions, but will not see if this commit has Tag linked to it.
Showing branch, tag details will give better overview about the commit

### What are the relevant issue numbers / Feature requests?
Feature request: http://feedback.gitlab.com/forums/176466-general/suggestions/3962044-show-tags-in-commit-view
### Screenshots
![gitlab_feature_show_tags](https://gitlab.com/uploads/haynes/gitlab-ce/771c1a36bb/gitlab_feature_show_tags.png)

@dblessing Can you take a look at this MR as well please?

See merge request !297

Developer can push to protected branch if allowed

#1876

See merge request !1418

Github importer: AJAX update status

#1907

See merge request !1417

Add action property to merge request hook