Refine character count limit semantics (#673).
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.

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase