Allow a Casebook to be tagged with one or more areas of interest by the author

[lizadaly]

From a controlled vocabulary, allow users to tag Casebooks with one or more areas of interest.

Tab 3 on this spreadsheet lists areas of interest that a Casebook could be tagged with.

• The controlled vocabulary should be a new type of model field
• The controlled vocabulary should be editable in the Django admin
• Adding or removing tags from Casebooks should be possible in the Django admin
• Adding or removing tags from Casebooks should be possible for the author on the front end (TBD where)
• It should (eventually) be possible to search on or browse through public Casebooks based on their tags

There's probably existing Django packages that do this, so look at some prior art and decide whether it's simpler to use a third-party package or roll our own.