Decide wording of buttons shown in grading mode

As part of our work on https://github.com/hypothesis/client/issues/1296, we discussed the wording of the button shown when a teacher is in grading mode and viewing annotations from the entire class:


@jaredpdesigns suggested this as an alternative:

The discussion around this so far:

- The button label should start with a verb, so it's clear that the button is an action and the user knows what it does.
- The name of the student no longer appears in the client - though it _does_ appear in the SpeedGrader interface. That's fine for Canvas users, but we'd need to keep this in mind when building grading integrations for other LMSes.
- We were looking for an alternative to including the user’s name in the button label was to prevent long names from wrapping and having the button appear taller than the default height.
- The term "selected student" isn't appropriate, because a "selected" object in the UI is something the user has selected, e.g. by clicking on. In this instance there is no selected student as such. The student who is currently being graded in the LMS is not really the "selected" student.
- A proposed alternative: Show all / Show student only

We need to decide:
- Will the button include the student's name?
- If the button doesn't include the student's name, what should it say instead?