RFC T157418: Trim whitespace in table cells, list items, headings

RFC T157418: Trim whitespace in table cells, list items, headings

* Matmarex had implemented this for wikitext headings in b3dd3881.
* This patch extends this to wikitext list items and wikitext table cells.
* Updated RELEASE NOTES.

tests/parser/parserTests.txt:
* All whitespace removed in output of list items, table cells, and
  headings. Removed corresponding whitespace in the input wikitext
  except for a few tests where the whitespace is significant "| +"
  or "| -", for example.
* Updated output of html/parsoid sections as well.
* Added new tests to spec white-space trimming behavior.

tests/phpunit/*:
* Fixed a few tests that used whitespace in list items and table cells.

Bug: T157418
Change-Id: I8ea34c7ab893c0c125c81d810feeb3c581e4bba1