Implement tree closing and opening #207
Description
homu has a command closed @bors treeclosed=<p> and @bors treeclosed-, which close and open the tree, respectively.
A closed for priority P tree means that no PR with priority less than P won't be attempted to be merged.
Bors can't currently do merges, but we could implement parsing of the command, checking of permissions etc. We will also need to store this information in the DB somewhere, because it is attached to a repository, rather than to a PR, so we will need to represent repositories in the DB somehow.
It would be great to check out how homu does this.