Make sure URLs are all lowercase

[Viper007Bond]

WordPress' URLs are case insensitive. We should enforce just one case type.

For example, both of these URLs work and display the same content even though they are different URLs:

​http://www.viper007bond.com/tests/
​http://www.viper007bond.com/TESTS/

[filosofo]

I saw a related problem last week: a client had entered his site and home URLs in camel case, but in some parts of the admin it was being printed in lower-case, leading Firefox to refuse to perform AJAX requests because it seemed like two different domains.

[ryan]

Moving enhancements to 2.8.

[janbrasna]

See #6627 and [9193] which was rolled back later.

[Denis-de-Bernardy]

Closing this as dup of #8038