- Add config variable `muc_show_logs_before_join`
- Restore nickname form for the original use-case (when not showing MAM logs before joining).
- Render nickname form in the `.muc-bottom-panel` when showing MAM logs (instead of in a modal)
- Show a message when there aren't any archived messages to show
- Fix converse-mam so that chat logs are properly fetched before entering a MUC

closes #1266