Remove columns storing predictable github endpoint URLs

the `contributors` table contains columns like: `gh_followers_url`, `gh_following_url`, gists, starred, subscriptions, organizations etc.

These URLS are returned by the github API and follow a predictable format.

We dont need to store that predictable surrounding text (i.e. `https://api.github.com/users/`) once for every record in the table. - it just wastes storage when we can compute on the fly if we need to hit these URLs

Some of them may deserve to stay, like `gh_avatar_url`, but we should be removing the vast majority of them.

I am working on a PR that will implement this on the fly recalculation it depends on some other refactoring of URLs though)