Support annotating the titles of Discourse topics

[tanius]

Our ethnographers using this software requested the ability to also be able to add annotations to the title of a Discourse topic.

This should be doable by adding one more annotator instance to the topic title element, and extending the way in which annotations are stored. Currently the target post is identified by topic number and post number, starting with 1. Perhaps simply use a post number of 0 to indicate that this is for coding the topic's title?

This is not an urgent feature to add, but when we got time and budget to do so, let's just add it.