[Volunteer Mobilization] Build tagging UI for documents

The system will need a UI for users to apply tags to portions of documents. Should free-form tagging be allowed, or should tags be considered a voting signal?

For example, free-form tagging would allow users to mark documents with things like: [interesting] [important] [critical], etc; creating a "Tag as commentary" environment.

On the other hand, pre-defined tags would present a series of tags for an entire document set, in which selection of a tag would be construed as a vote for that category on that tag. So clicking on the [important] tag would be seen as a vote for the portion of the document being "important" (tags defined by the questionnaire creator from issue #2)