Give task owner ability to select contexts to put in the loop by tag

[TristanThrush]

When a task owner uploads contexts via the rounds section of the task owner interface, they specify a tag for each context. For contexts which are already uploaded and in the db, let's display each tag with a check box component like we do for models. If a task owner checks the box next to a tag, then contexts with those tags will be put in the loop. If tags are unchecked then these contexts will be removed from the loop.