Make URL field a url type

[pdewouters]

Change URL Field to URL field type

Resolves #

I have:

• [ ] Run WordPress VIP PHPCS check and code parses without errors
• [ ] Added any new PHPUnit tests
• [ ] Run PHPUnit and all tests are passing after adding any necessary new tests
• [ ] Tested feature/bugfix on single and multisite install

@mikeselander

[pdewouters]

cc @dashaluna