[Fix #121] Eliminate unneeded AR dependency.

Since `ActiveRecord::RecordNotFound` is the only (known) error that can
be raised in `CurrentUserIdentity#validate_identifier`, using a blanket
`rescue` eliminates a direct dependency on AR. (We still inject a DAO
class into `#initialise`, but the only methods called on *that* are
`#find` and `#first`.)

RSpec: 772 examples, 0 failures
RuboCop: 174 files inspected, no offenses detected