Implement data sync functionality

[strogonoff]

Implement data synchronizer, that would let user integrate their changes and push to upstream Git / GitHub repo.

[strogonoff]

This is how new data sync screen looks at the point of latest release (it behaves as you would expect):

Currently, the app pushes directly to master. It doesn’t yet handle failed merges. If there are conflicting changes upstream, it will fail and will require the user to merge changes locally, inside app’s data repo (requiring Git CLI installation & familiarity with).

We have a few options:

• In case of conflicts in master we could start a branch with unique name, and push changes there. Then, a dedicated IT person could handle the merge (possibly using GitHub PRs).
• We could always operate in a unique branch for each user, and hence have no conflicts.
• We could operate on forks, but creating a fork automatically would require GitHub API integration, which doesn’t go smooth at the moment (primarily due to OAuth issues).

See https://github.com/ituob/itu-ob-editor/issues/11