Limit admin usernames to letters, numbers, underscores

We tried to do that, but apparently we failed!  Our failed check can lead to failures downstream.
(Solves https://github.com/learningequality/ka-lite/issues/870)

Testing:
* Tested with space ("a b") (now fails properly)
* Tested with leading 1 ("1a"), (continues to fail properly)
* Tested with "abc" - still succeeds properly.