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`:

##### After `WikiPipeline`:

See merge request !2324