Split the 'mediawiki.htmlform' module code into multiple files

Split the 'mediawiki.htmlform' module code into multiple files

This module implements several related, but separate enhancements to
HTMLForm. While it makes sense to use a single ResourceLoader module
to serve this code, it doesn't make sense to keep all of it in a
single file. It was approaching 500 lines of code, and pieces of the
separate features were mixed together.

This commit mostly shuffles code around, only tweaking some
indentation, 'var' statements and function wrappers.

There is one small functional change: the enhancements now use the
'htmlform.enhance' mw.hook, which is fired on document-ready and when
new fields are added dynamically. Previously it was only used to allow
extensions to define their own HTMLForm enhancements.

(Also moved HTMLForm styles into the same directory as the newly-split
scripts.)

Change-Id: I22054b39868239ddb59317dadfaaa067653f8804