Silently ignore < and > characters

- Add a test for this new behaviour.
- Change a site_has_hosts? test to make it test what it should test -
  leaving the angle bracket in the test data would have caused it to
  fail because it wasn't hitting site_has_hosts? because it was
  stripped out before and hosts.empty? would have returned true, when
  in fact we want it to return false as the default behaviour.
- This branch is a rework to fix this simple bug raised in pull request
  180 and not worry yet about the other issues raised in the PR's comments.

This stems from https://github.com/alphagov/transition/pull/180 and this only solves the original bug on Pivotal (https://www.pivotaltracker.com/story/show/64466526), not all of the other issues raised in the pull request discussion. Those other issues require changes that were out of scope of fixing this one bug and rather than half fixing some of them and not fixing others, I've made this work for now and then the other raised issues (URI vs. Addressable...) can be worked on as separate stories. Silently ignoring these characters that fail to parse with the current code is what we do in the other cases where things fail to parse.