Use "\z" instead of "$" to anchor validating regular expressions

Summary:
Via HackerOne. In regular expressions, "$" matches "end of input, or before terminating newline". This means that the expression `/^A$/` matches two strings: `"A"`, and `"A\n"`.

When we care about this, use `\z` instead, which matches "end of input" only.

This allowed registration of `"username\n"` and similar.

Test Plan:
  - Grepped codebase for all calls to `preg_match()` / `preg_match_all()`.
  - Fixed the ones where this seemed like it could have an impact.
  - Added and executed unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8516