Add experimenter buttons to tasks

[CCuskley]

As part of downex feedback, it's been requested that we add some experimenter controls to tasks. These should be implemented as a menu on the upper right that will pull out a "tab" with the following options:

• re-enter fullscreen (already in [Feature] Add button to re-enter fullscreen #407 )
• pause task (e.g., for break during administration)
• leave/end task and save responses (even if<80% completion

Once mocked up, we'll need to iterate these a bit and make decisions about specific behaviour (e.g., how/if to record pause etc)