[Scenario] Tag objects and tag repositories

This pull requests adds:

* A Tag user object type, which can be placed in an Activity to provide metadata about the activity
* A prototype "Summary View" which presents a pie chart of costs related to tagging; this is a prototype pending design or user requests
* A Tag Repository object type, which provide containers ("Mission Tags" and "User Tags") to make it easier to re-use specific tags. Exposed as top-level objects using nasa/mct#208
* A section in the info view which allows viewing, adding, and deleting tags with a plugin-specific user interface. Supported by nasa/mct#209