Remove existing whitespace stripping code

Since we’re doing this globally, we don’t need to handle it in a custom
way for the sign in form (and it’s much nicer encapsulated like this).

Also added some more extensive tests in this commit.

app/main/views/sign_in.py

@@ -30,8 +30,6 @@ def sign_in():
         return redirect(url_for('main.choose_service'))
 
     form = LoginForm()
-    if form.email_address.data:
-        form.email_address.data = form.email_address.data.strip()
 
     if form.validate_on_submit():