API: Labels validation in issues API

Skipping rc3 because somebody (me) created a tag but did not bump the
version to rc3.

Fix project description on explore page, fixes #7458

use default white-space for sidebar buttons, fixes #7549

Fix color toggle for labels in edit view

Fix label migration

Failure to create any labels at all caused this DB migration to run for over 40
minutes during a gitlab.com deploy this morning. This change specifies a color f
or each new label and strips invalid characters from the label name.

See merge request !1024

Add dealing with newlines for editor to 5.1-to-6.0

Fix env variable style for documentation

Add missing elements to webhook doc

Fix minor spelling error in repo settings page

API: Fix labels return code

Update gems with errors, to prevent 500 errors

Consistency: Capitalize hex values of sugested colors

Fix Md style for API.

Fix Md style for projects API doc.

Fix grammar and spelling

Consistency, spelling and grammar

Label permissions

* Update label permission docs
* Add authorisation to labels via API

See merge request !1020

Use new git max_size in the settings initializer

This allows us to avoid repeating default max_size setting in the omnibus-gitlab repository.

See merge request !1021

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