Make wysiwig character counter optional (#673).

Parameterize via flag passed to getEditor.
Merge branch 'develop' of https://github.com/PitonFoundation/atlas into 673-character-count-ubiquitous-in-builder
Move char limit config out of template (#673).

RichTextEditorMixin character count limit can be set as needed via
getEditor param, rather than in template source.
Refine character count limit semantics (#673).

- Remove default character count limit. Require specific count limit or
none when calling getEditor() on RichTextEditorMixin.

- Optimize site of argument validation.
Merge pull request #758 from patternleaf/673-character-count-ubiquitous-in-builder

Parameterize character counter in builder WYSIWYG editors (#673)
Update changelog to reflect fix for #673