Move CSS styles out of the options display callback to a separate CSS file.

Enqueue the new profile-options.css stylesheet only on profile.php.
Ensure all new strings are given the proper textdomain of 'wpsm'.

Also do a bit of cleanup:
* Define the public $sessions var in the profile options display callback
* Move an inline-style to the profile-options stylesheet
* Simplify the nooped plural string lines