Since we already have perfectly sensible logic for validating email addresses,
let's leverage that and simplify the logic while we're at it.
Emails with spaces are no longer permitted (why were they?)
We had quite a few cases in core where inputs are being mutated as a side
effect of calling a method.
This handles all the cases where specs caught this.
Mutating inputs makes code harder to reason about. Eg:
```
frog = "frog"
jump(frog)
puts frog
"fly" # ?????
```
This commit is part of a followup commit that adds # frozen_string_literal
to all our specs.
