Allow annotators to assign motivations to an annotation; fixes #57

[cbeer]

Pending some of the refactoring in #61

[ggeisler]

I know I didn't include these in my previous mockups on this issue, but I wonder if we could add Translating and Transcribing to the set of motivation options?

Those have come up as real use case motivations with at least one of our current Mise stakeholders, and more generally, they seem to motivations likely to be frequently used by those with manuscript use cases. So it might help advertise the feature better if we include motivation options that are likely to resonate with potential users.

[cbeer]

I think your mockups used terms from the web annotations spec: https://www.w3.org/TR/annotation-model/#motivation-and-purpose

I'm not sure what the process is for registering new motivations in https://iiif.io/api/registry/motivations/ (the linked process is helpfully blank https://iiif.io/api/registry/process/), but I think we'd want to see them there before baking them in by default (although downstream applications should be able to add whatever motivations they want..)

[ggeisler]

Oh, right, I remember that now after seeing the page again... Sure, that makes sense.

It looks like nothing has been registered, and as you note, they don't actually provide the promised process for registering. I can maybe pursue this through Slack or something soon. Maybe it'll help once we have something working to show how the motivations can be used, and then I can point out (or at least ask and see if my perception is accurate) that a lot of researchers in the community might be interested in transcribing and translating.