Store version of merge bot in deployment

We should know what version of mergit is currently deployed at all times.

Maybe instead of making just the `mergit` binary a CI artifact, make a `mergit-bundle.tar.gz` bundle that contains the `mergit` binary and a text file with the current commit. Then uploading that bundle in the merge bot deployment would maintain a file on the EC2 instance with the commit stored in it.

Possibly also show it in the web UI

Related: #194

Trace the CI artifact creation and merge bot deployment mentioned in the issue, including how the bundle reaches the EC2 instance. Determine how the deployed commit would be stored and identify whether the web UI is in scope. Done means the deployed instance retains a file identifying the currently deployed mergit commit.

Written by the indexing model from the issue text.