Allow editing extra_capabilities

[mythmon]

With the new capabilities work landed in Normandy, there is a new editable field that should be accessible: extra_capabilities. It is logically a set of strings. Though since JSON doesn't have sets the type is actually Array<String>.

Right now there aren't any further semantics associated with this field, we just need to be able to type a list of strings in.

[mythmon]

For extra fanciness, this could autocomplete from the list of capabilities the server knows about, which there is an API endpoint for now. We should still allow free-form text as well though, since some valid capabilities might not be known by the server.