Support bottom-aligned scroll panels.
Clean up chat input.

- Team chat by default; use tab key or button to change.
- Removed "Use RETURN key..." prompt.
- Remove jarring transition between display and entry modes.
- Scroll to bottom on opening entry mode.
- Disable any/team toggle for spectators.
- Minor spacing/padding tweaks.
- Code cleanup.
Use the newly polished text entry in C&C.
Merge pull request #3983 from pchote/polish-chat-input

Polish chat input.