The check mark which appears for valid input in assorted forms
(such as login and registration) didn't have alternative text
for better accessibility. Added "Valid" as the alt text in all
places it's used.
Fixes#4876.
This completes a major redesign of the Zulip login and registration
pages, making them look much more slick and modern.
Major features include:
* Display of the realm name, description and icon on the login page
and registration pages in the subdomains case.
* Much slicker looking buttons and input fields.
* A new overall style for the exterior of these portico pages.
Jeremy Bowman
Brock Whittaker
adnrs96
Tim Abbott
Rishi Gupta
Umair Khan