Closes #13905

Prior, if the user enabled 2FA, then disabled it and came back some time
after the grace period expired, they would be forced to enable 2FA
immediately.

The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it

This ensures that if a project is later re-created using the old path it
doesn't end up re-using the old cache. This also ensures we don't keep
the cache around until its expired by Redis itself.

Fixes gitlab-org/gitlab-ce#13790

This ensures that the emptiness cache (used for Repository#empty? and
Repository#has_visible_content?) is flushed after comitting changes
(using the web editor, API or Git) for new repositories. Once a
repository is no longer empty there's no need to explicitly flush the
cache for Repository#empty?. The cache for
Repository#has_visible_content? in turn is already flushed whenever
needed.

Fixes gitlab-org/gitlab-ce#13387

Discussion:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2560#note_3659905

Also added API specs for that

+ minor improvements

This allows us to take advantage of Rails' `to_partial_path` to render
the correct partial based on the Blob type, rather than cluttering the
view with conditionals.

It also allows (and will allow in the future) better encapsulation for
Blob-related logic which makes sense for our Rails app but might not
make as much sense for the core `gitlab_git` library, such as detecting
if the blob is an SVG.

If path_with_namespace is nil Repository#raw_repository will also return
nil. Apparently code out there creates a Repository instance without a
namespace path. Right.