fixed nav bar for empty repos (fixes #4375)

Adding detailed documentation of the markdown syntax.

Closes #4331

Closes #4424

This includes standard markdown as well as GitLab extended markdown as well as references to documents used.

Create uploads dir otherwise backup fails. Fix #4304

Gravatar image fixes when gravatar unavailable

Change-Id: Iad49f225f23675b843457df5d6965c7cd50e99a3

-Alt text to image was set to hash, which would make the page display hashes if the image/gravatar  wasn't available, set the alt text to '', and made the main tool bar image follow the same or similar constraints as the other images (.avatar, .s26)

Conflicts:
	app/assets/stylesheets/themes/ui_bb.scss
	app/views/snippets/_snippet.html.haml

Change-Id: I42e1e86f535486fd2556d4b3e4591f3676eea033

Rename unspecified to make it clearer and in line with scrum terminology

fix bad constant reference in `project_routing_spec.rb`.

whitespaces in gitlab.yml.example

Add missing assigned_to renaming to merge_requests_load_context

we should not use `console.log` in production code...

Notify email to mentioned users; show participants ala github on issuables

Database seeding failing

[Doc] Update 5.0 to 5.1: Recompile assets

Fix wrong file versions in update docs (5.2 to 5.3)

add db index for authentication_token column in users table