settings: Added setting to turn on and off realm name in email subject.

Users having only account in one realm will not be distracted by realm
name in subject lines of every email.  Users who have multiple
accounts in realms can turn this setting on and receive a
corresponding realm name in email's subject.

Tweaked by tabbott to rebase and address a few small issues.

Fixes #5489.

zerver/lib/send_email.py

@@ -41,7 +41,6 @@ def build_email(template_prefix: str, to_user_id: Optional[int]=None,
         context = {}
 
     context.update({
-        'realm_name_in_notifications': False,
         'support_email': FromAddress.SUPPORT,
         'email_images_base_uri': settings.ROOT_DOMAIN_URI + '/static/images/emails',
         'physical_address': settings.PHYSICAL_ADDRESS,