Signed-off-on front-end portion of toc level limit.

This includes the signed-off-on part of:

https://bugzilla.mozilla.org/show_bug.cgi?id=845355

The "customize" link is hidden for now.

What we still need is the server-side portion of this, migration and value handling.  My thought was:

*  Change the current show_toc field from Boolean to Integer
*  The value scheme would be:

0 = No TOC
1 = TOC, show all headings
2 = TOC, show only H2 headings
3 = TOC, show only H2 and H3 headings
4 = TOC, show only H2, H3, and H4 headings

4 would be the default.