Fix bug 823733: Rework RevisionForm save logic for section editing.

This adds a separate code path (largely because I suspect we're going
to find more things that need to be different in section edits) for
saving a section edit, and ensures that the existing show_toc and
review_tags are preserved (since they're not submitted in a section
edit).

This is also a non-breaking change; all existing unit tests continue to pass, for example, and no current functionality is removed or changed (meaning we can just deploy this without needing to waffle-flag it).