Repos: allow config to exclude certain repos

[danielbeardsley]

Problem

Using an Org-level webhook is really convenient when you have lots of repos. However we have no way to hide certain repos.

Do This

Add a config option to hide certain repos (maybe an array of strings or regexes?) by default. Effectively, make the "repos" dropdown filter default to having some repos unselected.

CC @sterlinghirsh

[mlahargou]

We could have used this today because we don't want a particular repo to show up on pulldasher.