Fix the `normalize` and `array_attribute` macros conflicting with each
other on the WikiPage model. This meant code like
`wiki_page.other_names = "foo bar"` didn't work. Both macros defined a
`other_names=` method, but one method overrode the other.
The fix is to use anonymous modules and prepend so we can chain method
calls with super.
* Introduce an abstraction for normalizing attributes. Very loosely
modeled after https://github.com/fnando/normalize_attributes.
* Normalize wiki bodies to Unicode NFC form.
* Normalize Unicode space characters in wiki bodies (strip zero width
spaces, normalize line endings to CRLF, normalize Unicode spaces to
ASCII spaces).
* Trim spaces from the start and end of wiki page bodies. This may cause
wiki page diffs to show spaces being removed even when the user didn't
explicitly remove the spaces themselves.
