Save subject page body to Tag.body when tags are created

[jimchamp]

Addresses: #9685

Makes the following changes to the subject page editing flow:

• Saves the subject key to the fkey field when a tag is created from a /subjects/* page
• Stores the subject page HTML on new Tag as body
• Adds a "Page Body" textarea to the tag form inputs
• Patrons are redirected to the /tags/* page when they navigate to a subject page that is linked to a Tag
• Updates subject page template edit button's copy to "Create Tag" when no linked tag is found

Limitations

• The "Page Body" field is not pre-populated with the subject page HTML

Technical

fkey is meant to be immutable, and is displayed in a read-only input.

Testing

Try the following:

1. Navigate to a subject page that has no associated Tag. Ensure that there is a "Create Tag" button in the upper-right corner of the content body.
2. Click the "Create Tag" button. Leaving the "Page Body" field blank, submit the form.
3. Verify that you are viewing a /tags/* page, and that the subject page's body is being rendered.
4. Attempt to navigate back to the original subject page. Verify that you are redirected to the linked Tag page.

Screenshot

The "Create Tag" button

Stakeholders

[jimchamp]

I think that we do want this somewhere on the page, just not in a heading element (and maybe somewhere else on the page).

Maybe displaying this before tag.body in /type/tag/view.html is the right spot? This would ensure that:

1. The description is always present in the tag's view
2. The description only appears on the page once when tag.body is a rendered subject page

[jimchamp]

Labeled Needs: Submitter Input Waiting on input from the creator of the issue/pr [managed] until the following are addressed:

• Bug: Bad Request error is returned on Tag edit
• Flow: Librarians given the opportunity to edit the tag before it is created (reinstate edit view on tag creation)
• UI: Change "Promote to Tag" button copy to "Edit"
• Schema: Tags of type subject should have a sub-type (subject, people, places, times, etc.)
• UX: Pre-populate initial edit form with the /subject/{subject} page's HTML

The last item will likely require that the conditional logic that was added to the "Edit" button is removed (i.e. the button's href and copy should be the same, regardless of whether a tag is linked to the subject page).