Remove/rename all usages of `gitAccount`

[jescalada]

As per #973, we no longer associate commits by gitAccount (which is actually the GH username). #1042 further extends support for other Git hosts, which means that storing GH usernames specifically doesn't make sense either.

We'd like to remove or rename all usages of gitAccount in a way that makes sense considering that repos from different hosts (GitHub, GitLab, etc.) can be present. We should also take care of not breaking anything that used to rely on the gitAccount - though most of the extant usages and logic are in the UI.

Finally, we want to adjust the UI (especially src/ui/views/User) so that it suits the current implementation.

Additional context
Related PRs #973, #1042, #1257

[andypols]

We may need this for automating things like raising issues and creating PRs from within GitHub.

[kriswest]

@andypols @jescalada it may be needed for additional github/gitlab functionality, but it should probably take a different form than it does currently (perhaps based on an additional table or at least a JSON field containing multiple values, so that multiple accounts at different SCM providers can be associated with a single user). A migration would be needed to avoid users having to re-enter the data of course...