Make annotations templatable

[guswelter]

A single annotation form structure is currently hard-coded into the viewer. Generalize this such that any annotation template structure can be specified (i.e. make it possible to specify any set of field names, types, and value options).

The current implementation is:

1. User completes annotation form input and clicks Create
2. Client (JS) code reads the values into a class instance which represents the annotation on the client side
3. Client code submits the data object as json to the backend server.
4. Server stored the annotation in the database such that the label (form field values) is stored as json in a single database column, and other things like file, series, and start & stop times are represented as columns in the database.

And also, there is obviousy the corresponding requisite functionality for annotation retrieval, rendering, and updating.

Because the backend treats the label (all form data) as a single json string and has no specific functionality (to my recollection) for what is inside the label (the form structure), it is only the client-side javascript code which needs to be generalized.