Don't allow the authoring of an annotation in a group the user is not authorized to write to

This issue is only applicable to restricted groups—the only case at present that a Hypothesis user may see content in a group but not write to that group.

This issue applies in the case that:

* A user is viewing content within a restricted group (i.e. the restricted group is the current "focused", or selected, group)
* The user is logged in but is not a member of the restricted group
* The user highlights some text and clicks "Annotate"

## Current Behavior

The client will allow the creation and population of a new annotation, and the publish control (button) is enabled. However, when the user submits the new annotation:

The backend is (correctly) responding with an error because this user is not authorized to write annotations to this group.

## Expected Behavior

Needs clarification. What _should_ happen in this scenario? My hunch is that this was a loose end during the groups implementation phase that wasn't tied up. Some possible options (not vetted for awesomeness) include:

* An error message
* Disabled post button
* Changing focused group on the author's behalf
* Disabling the `Annotate` button in the annotator toolbar

None of those sound like great ideas immediately, at least in isolation.