Make Subjects editable

We don’t allow editing of Subjects. This prevents easier categorisation of, mainly, Grid uploads (unless they had metadata added externally). We should allow editing, including batch editing, inclusion in templates etc. We may need a new UI (multiple choice from a finite list).

No files, tests, or entry points are named. Start by tracing the existing Subject metadata, editing, batch-editing, and template flows, then determine how the finite list should be represented in the UI. Done means Subjects can be edited individually and in batches, and are supported in templates.

Written by the indexing model from the issue text.