Rewrite OriginsField to use a custom validator

[mattrobenolt]

This will allow us to wrap up all of the logic in one place instead of treating it like a URLField.

This will let us solve some of the issues around weird urls that happen from raven.js, like chrome-extension://.

See: getsentry/sentry-javascript#157

[mattrobenolt]

Need to look at this again to see if it's still a problem.

[mack7121]

If this is still an issue, I want to help.

[dcramer]

Doesn't look like this is still a problem, as we have simple custom URL parsing here.