Before, the styling for `img:not(.emoji)`` was repeated between
`.md:not(.use-csslab):not(.wiki)` (for comments and the Markdown
preview), `.documentation.wiki` (for help pages), and
`.issuable-details .description .wiki` (for MR and issue descriptions).

In these places, we now simply use `.md`. Wherever we use `.md` but
don't want this image styling (like wikis and blobs), `.md-file` is
added.

Mousetrap is used as the help-tool to listen to keystrokes

Added currentDiffIndex getter to store that holds
the index of currently active diff file in the list

Instead of computing it on the component, we will take advantage of it
being available for all components in DiffsApp

Testing keyboard navigation and jumpToFile()

The pre-release tags are set at the beginning of a release, so it
would be impossible to figure out which version was running if we're
running nightlies.

In that case it's better to still link to the SHA. These versions
don't get deployed to .com.

This will result in a 404 when running an unreleased security patch
while still allowing users to find the commit when it is made available.

Rename btn-new, btn-create and btn-save to btn-success

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>