Add HTML form validation for the registration form (#14560)

* Add HTML-level validation of username in sign-up form

* Make required fields with incorrect values more visible

* Enable HTML form validation for the registration form

* Mark agreement checkbox as required client-side

* Add minimum length to password

* Add client-side password confirmation validation

app/javascript/styles/mastodon/forms.scss

@@ -364,7 +364,8 @@ code {
       box-shadow: none;
     }
 
-    &:focus:invalid:not(:placeholder-shown) {
+    &:focus:invalid:not(:placeholder-shown),
+    &:required:invalid:not(:placeholder-shown) {
       border-color: lighten($error-red, 12%);
     }