Import GitHub wiki into GitLab

Closes #2834 

GitHub doesn’t apply any constraints to theirs wiki slug allowing chars like ,, :, *, etc, we need to remove our constraints or some wiki pages will not be available after they are imported.

Some wikis use the Gollum's tags to link its internal/external resources like: images, urls, wiki pages, etc. So, to avoid that wiki links/images displays completely broken after they were imported, we added a new `WikiPipeline`, that for now will parse only simple links, and image tags.

##### Before `WikiPipeline`:

![Screenshot_2016-01-11_20.14.48](/uploads/46fd5dbb5acfc70aa8ecca3050b675e4/Screenshot_2016-01-11_20.14.48.png)

##### After `WikiPipeline`:

![Screenshot_2016-01-11_20.15.56](/uploads/b1d94aa852f385f867a7868c358b4257/Screenshot_2016-01-11_20.15.56.png)

See merge request !2324