gitserver: update janitor so it sets repo sizes
Created by: sashaostrikov
Gitserver janitor during its first run will now query the DB for repos in gitserver_repos table which have repo_size_bytes = NULL and update them with calculated values. Janitor always calculates a size of a repo (i.e. directory in file system) and now these data are used to fill the gaps in gitserver repo sizes.
Closes https://github.com/sourcegraph/sourcegraph/issues/33000
Test plan
Unit tests are added