URLs must be for Github PRs

[davearonson]

As a user,
I want to have other users limited to putting in valid Github pull request URLs,
So that I can find actual PRs instead of spam.

Given that I am on the New Pull Request page,
When I enter a URL that is NOT for a Github pull request,
And I submit it,
Then I should get an error message
And the URL should not be in the system.

Given that I am on the New Pull Request page,
When I enter a URL that IS for a Github pull request,
And I submit it,
Then I should NOT get an error message
And the URL should be in the system.

[davearonson]

Added a regex-based validation, and test with a canonical and an invalid url.

[davearonson]

Duh, should also test w/ some other good formats, and maybe give some instruction.

[davearonson]

Done. Still not accepting w/o a protocol on them tho. Most will probably be cut and pasted from browser address bars, and should thus have protocols.