Replace references to master/slave

The use of master/slave terminology has been deprecated for good reasons, with several projects like Rust, Django and the Linux kernel having already moved away from it. Geary should catch up.

• Cut a new branch mainline off of master, push to the repo
• Set mainline to be the default and protected in Gitlab project settings
• Update branch in Damned Lies (Infrastructure/damned-lies#143)
• Update Flatpak manifest to use new mainline branch name (!191 (merged))
• Update app file in gnome-apps-nightly (GNOME/gnome-apps-nightly!13)
• Check the code base, rename any instances there (!192 (merged))
• Update wiki pages
• Update all outstanding merge requests targeting master to target mainline instead
• Delete master branch in Gitlab (Infrastructure/Infrastructure#127 (closed))
• Mention the change on the list & IRC channel (and gnome-i18n?)