Fix disabled login/register buttons after failed attempt

In the login window, when the user enter a wrong email or password, the
"Log into my ... account" button used to remain disabled, which was very
confusing for users.

Steps to reproduce:
- Go to /login
- Type wrong email/password combination
- Press "Log into..." button

Similarly, users could not click the "register button after a failed
register attempt. For example, this occurred to users with an invalid
email address, such as "fooatgmaildotcom".

lms/templates/login.html

@@ -100,7 +100,7 @@ from third_party_auth import provider, pipeline
         $submitButton.
           removeClass('is-disabled').
           attr('aria-disabled', false).
-          removeProp('disabled').
+          prop('disabled', false).
           html("${_('Log into My {platform_name} Account').format(platform_name=platform_name)} <span class='orn-plus'>+</span> ${_('Access My Courses')}");
       }
       else {

lms/templates/register.html

@@ -85,7 +85,7 @@ import calendar
         $submitButton.
           removeClass('is-disabled').
           attr('aria-disabled', false).
-          removeProp('disabled').
+          prop('disabled', false).
           html("${_('Create My {platform_name} Account').format(platform_name=platform_name)}");
       }
       else {