Also specify separate project factory for project with filled repo

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.

Fixes #2166

Fixes #2617

Also fixes the spec so that it actually tests the thing it says it's
testing. Hooray!

Closes #1308

Forgot they were so big!

Should now be much clearer about what each spec is actually testing.
For example, instead of testing stuff like link classes and titles in
every single call, we only test those things once, in their own specs.

Didn't bother with files in db/, config/, or features/

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