Fix rce handling of empty <ul>
Fix rce handling of empty <ul>

fixes COREFE-190

- our tinymce config included plain old html elements in the list of
extended_valid_elements. Because they didn't include the "-" flag,
they were not permitted to be empty, and were removed from the content
as it was being parsed on saving. This broke handling out of order
li & ul.  I removed all them, except where attribute lists were
specified, and in the case of `ul[compact]`, I added the '-' prefix
so it would remain in the markup.

test plan:
  - in an rce, flip to html mode
  - insert "<li>whatever</li><ul></ul>"
  - flip to editor mode
  > expect it to work and not throw an exception

Change-Id: I3cc3dc2c93ebd932e3d5e085112248daa3068258
Reviewed-on: https://gerrit.instructure.com/203667
Tested-by: Jenkins
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Reviewed-by: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms