* First lame pass at adding optional git commit hash display on /about/more page.

Currently, this is implemented by checking for the existence of a file called CURRENT_RELEASE in the home directory of the user running Mastodon. If the file exists, its contents are added.

I've modified my update process to include the following before precompiling assets:

git log -1 | head -n 1 | cut -d " " -f2 > ~/CURRENT_RELEASE

That puts the current commit hash into the file ~/CURRENT_RELEASE, but you figured that out because you're a smart cookie.

As I am quite sure this is a horrible methodology for implementing this, I look forward to any improvements you have to offer!

* Updated to handle instances that share a user - the CURRENT_RELEASE file now lives in the instance's base directory.

This also requires modifying the update hook to `git log -1 | head -n 1 | cut -d " " -f2 > CURRENT_RELEASE`

* Adds version.

* Cleans up code.

* Removes standalone endpoint and adds version to instance endpoint.

* Addresses feedback from #2181.