v3.0: add extension popup menu to customise extension behaviour
- Dropdown Menu when Extension Icon is clicked (resolves #4)
- Enable/disable extension
- Enable/disable autohiding servers
- Enable/disable autohiding channels
- Specify small window width that servers/channels will autohide
- Donate/Support button
- Remember whether servers list is hidden/shown in the Discord UI (resolves #5)
- Only applies when server autohide is disabled