switch from master to main everywhere

back in gitlab#75 (closed), we switched the default branch name in GitLab from master to main. there's a good rationale of "why" there.

this was done before the RFC process was created, and, as such, failed to actually establish policy. so much so that we still actually have repos with master as a HEAD branch name, and this is causing some confusion. the most notable example i can think of now is fabric-tasks, but the wiki-replica repo also suffers from that problem (although that's tied to a limitation in gitlab, that doesn't allow changing the wiki branch names).

let's make a cleaner proposal for this going forward.