HTMLForm entity labels are now optional and escaped

Things like checkboxes have no label, yet a label div gets generated
anyway. This is annoying when maybe I don't want that empty div hanging
around (i.e., it looks like it's part of other option groups when I
have left margins on all .mw-input).

This patch will now also escape 'label' fields by default. For the old
functionality you must now explicitly use the 'label-raw' field.

Change-Id: I8f8340911b7495a91c93e7f2eb7c041b2a7f2179
HTMLTextAreaField: Allow sizes to be overridden by child classes

Change-Id: I51e8bcba95c7b555f072329b01c6577115f246b3