[ ] Search for a keyword in contributions, and add a node at the bottom of the tree pane showing recent searches and containing bottom-level discourse-parts whose contributions contain that keyword. Add to a search bar in the form "with-word:XXXXX"
[ ] Search for particular users, with a similar interface. Right-click a username in a contribution to add "username:XXXXXX" to a search bar (or type that directly)
[ ] Search results can be selected just like discourse parts, and included in queries
[ ] Search query strings are stored in server attributes. They can be deleted.