Add support for room topics

[benbatt]

Adds support for setting a room server topic via a new set topic command.

• The topic is saved in a new /room_prefs file
• A new topic_timestamp member variable is used to determine which clients have seen the current topic
• For clients that haven't seen the current topic, MyMesh::loop() pushes the topic before any new posts
• The topic is pushed as a post from the room server itself
  • This seems like it should be backwards compatible
  • Maybe the app could be updated to display the latest post from the room server as a persistent header in the room UI?

[benbatt]

This implements a version of #2122.