Adds a rake task which can be used for removing EXIF
data from existing uploads.

Updated Rake-specific documentation to include storage rollback,
and improved migration and rollback instructions.

Markdown renderers find it easier to determine
where lists start and end when lists are surrounded
by new lines.

For consistency, also ensure entries in the list
are aligned when they span multipls lines.

- Also makes other minor Markdown fixes that were near the main fixes.

Also, since Gitaly now takes care of checking for storage paths
existence/accessibility, we can remove those check from the
gitlab:gitlab_shell_check task and advance further into 0 direct disk
approach on gitlab-rails