This adds an organization description field to the Realm model, as well as
an input field to the organization settings template. Added three tests.
Set the max length of the field to 100 characters.
Fixes#3962.
On reloading the page after disabling email changes does not check
the "Prevent users from changing their email address".
Adding realm_email_changes_disabled to page_params_core_fields fixes the problem.
Change applies to both subdomains and non-subdomains case, though we use
just the EXTERNAL_HOST in the non-subdomains case if there is only 1 realm.
Fixes#3903.