Got around to 'guest registers as member' feature spec.

Also added validation to `PermissiveUserBuilder#execute` so that error
messages are available for display. (Getting them actually displayed
should be the *next* trick.) One typo in the `#validate` method also
cleaned up; return value is immaterial there, only whether anything has
been added to the error list.

`UsersController` private interface cleaned up a bit; previous method
`#persist_user_and_move_on` had been reduced to a one-liner, so replaced
its call (in #create) with that one line.

RSpec: 177 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 54 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 4