Fully overhaul/redesign mentionable text input with new mention popup

[kevinaboos]

• Support mentioning users (and notifying the room) with '@'
• Support selecting a room or space to link to with '#' (yes, spaces too!)
• Support selection a slash command with '/'
• Mention popup now supports dynamic resizing, both above and beneath the text input, as well as keyboard navigation and mouse click or touch drag/tap selection
• The mention popup is now an overlay so it can expand beyond the bounds of a smaller RoomScreen, and also shrinks to fit the app window on mobile or on a smaller app window
• Mention popup shows nice messages for loading or no matches found
• MentionableTextInputs now save and restore their own state
• Remove the outdated and inefficient RoomScreenProps approach in favor of a vastly more efficient set_room_context(), which is only set when the room actually changes.
• Remove the old CommandTextInput which we don't need to use any more
• All query matching and sorting/ranking is done off the main UI thread, on a sync bg thread for the set of users, and an async bg task for rooms/spaces
• Make RoomScreen event handling way faster for background un-init'd rooms
• Editing a message now drops mentions removed from the text and adds new mentions too.
• The /html slash command now sends a proper plaintext fallback that strips HTML tags and decodes/unescapes HTML entities

Supersedes and closes #816
Supersedes and closes #573
Closes #451
Closes #506
Closes #615
Closes #617
Closes #452