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).
Merge pull request #800 from ubernostrum/section-edit-toc

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