Replace current Wiki system with Gollum Wikis.

This commit replaces the old database backed Wiki system with the
excellent Gollum git based Wiki system.

The UI has been updated to allow for utilizing the extra features
that Gollum provides. Specifically:

* Edit page now allows you to choose the content format.
* Edit page allows you to provide a commit message for the change.
* History page now shows Format, Commit Message, and Commit Hash.
* A new Git Access page has been added with the Wiki Repo URL.
* The default page has been changed to Home from Index to match
the Gollum standard.

The old Wiki model has been left in tact to provide for the
development of a migration script that will move all content stored
in the old Wiki system into new Gollum Wikis.
Add Spinach coverage for Gollum Wiki system and correct the Delete link.

The previously failing Spinach steps have been fixed with this commit.
I have also added new steps that cover the entire usage of the Wiki
system.

The new Spinach steps revealed a minor bug in the Delete page process.
The path for the "Delete this page" button was previously set to
`project_wikis_page(@project, @wiki)` when it should have been
using the singular `project_wiki_page(@project, @wiki)` path helper.

The link has been corrected and all steps are now passing.
Create Wiki migration task.

This commit adds a new Rake task for migrating all of your existing
Wiki content from your database into new Gollum repositories.

The bulk of the logic happens within the `WikiToGollumMigrator`
class which is decently test covered and located in the lib directory.

The new Rake task can be executed by running:

   `bundle exec rake gitlab:wiki:migrate`

It will output a nice log of every project that it migrates along
with success or failure messages.

I have used it on my own installation to migrate my Wikis successfully.
Use Gitlab Markdown for Markdown files and Gollum to render the rest.

This commit enables the usage of the Gitlab Markdown post processing
on all Markdown formatted files. For file types that do not contain
Markdown, it defaults to the Gollum native renderer to process the
content.
Limit available Wiki formats to Markdown and RDoc.
Fix whitespace on MARKUPS constant in GollumWiki class.
Add settings tab to project. Move all project administration there
Adds DB-sensitive tablename escape for backup task.

Uses ActiveRecord::Base.connection.quote_table_name.
Fixes issue 2437.
Remove tree single-tab panel
Change spinach tab tests to fit new tabs navigation
Merge wiki, wall & snippets under one tab - writeboards
add resize button for better wiki reading
rename writeboards to wiki
on wall subpage active tab should be wiki too
Use wiki tab for wiki, wall, snippets
Merge branch 'feature/writeboard'
add dark solarized theme for code preview
finish up with solarized dark
reannotated
restyle header a bit