Rename ENV['PROTOCOL'] to ENV['GL_PROTOCOL'] to conform to what GitLab Shell expects and make the `protocol` param in `GitAccess` mandatory.

If a site specifies a relative URL root, emoji files would omit the path from
the URL, leading to lots of 404s.

Closes #15642

This stands as an alternative to using OAuth to access a user's Github
repositories.  This is setup in such a way that it can be used without OAuth
configuration.

From a UI perspective, the how to import modal has been replaced by a full
page, which includes a form for posting a personal access token back to the
Import::GithubController.

If the user has logged in via GitHub, skip the Personal Access Token and go
directly to Github for an access token via OAuth.

IssueReferenceFilter will end up processing internal issue references
when a project uses an external issues tracker while still using
internal issue references (in the form of `#\d+`). This commit ensures
that these links are rendered as external issue links, regardless of
whether the project one currently views uses an internal or external
issues tracker.

Fixes gitlab-org/gitlab-ce#19036, gitlab-com/performance#16

If Redis were not running or USE_DB were set to false, the
application settings retrieval would fail completely. This
change only attempts to use the cache if the system actually
wants to connect to the DB and rescues any failures in talking to
Redis.

Closes #17557

No reason to split it into a separate gem when the gem barely did anything. We can use gemojione directly, making updating gemojione that much easier.

Also fix the Rake task and update gemojione to 2.6.1. This adds the EmojiOne Spring update.

Changelog: https://github.com/jonathanwiesel/gemojione/blob/master/CHANGELOG.md

Fixed privilege escalation issue where manually set external users would be reverted back to internal users if they logged in via OAuth and that provider was not in the `external_providers` list.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12744656