fix(client): Allow leading/trailing whitespace on email addresses.

fixes #519
Standardize on the parameter name for an element - change `which` and `selector` to `el`
cleanup: Add more functional tests to ensure users that sign up with leading/trailing whitespace on their email address can sign in without the whitespace.
Merge pull request #1797 from mozilla/issue-519-whitespace-email

fix(client): Allow leading/trailing whitespace on email addresses

fixes #1803