Add navbar for logged in users w/ link to profile page
Rewrite signup page with WTForms
Update unit tests for new signup form

I disable the CSRF feature of wtforms in the unit tests since it is unnecessary
for our tests to retrieve and then set the correct CSRF token each time. If the
feature is left enabled, the tests fail.
Convert login form to use WTForms

Standarizes validation code convention (using validates_). Update unit tests to
fit new behavior.