milestones and issue templates #20

Labels and assignees exist but nothing groups work toward a release and nothing structures intake. Milestones: create/close, attach issues/MRs, progress on the web. Templates: .gitbay/issue-template*.md surfaced by the CLI ($EDITOR prefill) and the web form.

Shipped in c813bd2 and live. Milestones: milestone create/list/close/reopen, issue milestone / mr milestone attach (title or none), progress (closed+merged vs open) on /owner/name/milestones with bars, links on issue/MR pages. Templates: .gitbay/issue-template*.md at the default branch — issue templates lists them, gitbay issue create prefills $EDITOR with the default, and the new web form (GET issues/new, accounts mode) prefills with a template picker. Created v0.2.0 on this repo as the first real milestone.