templates: Add new context variables to all templates.

This adds a few new helpful context variables that we can use to
compute URLs in all of our templates:
* external_uri_scheme: http(s)://
* server_uri: The base URL for the server's canonical name
* realm_uri: The base URL for the user's realm

This is preparatory work for making realm_uri != server_uri when we
add support for subdomains.

zerver/context_processors.py

@@ -9,7 +9,14 @@ from zproject.backends import (password_auth_enabled, dev_auth_enabled,
 
 def add_settings(request):
     # type: (HttpRequest) -> Dict[str, Any]
-    realm = request.user.realm if hasattr(request.user, "realm") else None
+    if hasattr(request.user, "realm"):
+        realm = request.user.realm
+        realm_uri = realm.uri
+    else:
+        realm = None
+        # TODO: Figure out how to add an assertion that this is not used
+        realm_uri = settings.SERVER_URI
+
     return {
         'custom_logo_url':           settings.CUSTOM_LOGO_URL,
         'register_link_disabled':    settings.REGISTER_LINK_DISABLED,
@@ -20,7 +27,10 @@ def add_settings(request):
         'only_sso':                  settings.ONLY_SSO,
         'external_api_path':         settings.EXTERNAL_API_PATH,
         'external_api_uri':          settings.EXTERNAL_API_URI,
+        'external_host':             settings.EXTERNAL_HOST,
         'external_uri_scheme':       settings.EXTERNAL_URI_SCHEME,
+        'realm_uri':                 realm_uri,
+        'server_uri':                settings.SERVER_URI,
         'api_site_required':         settings.EXTERNAL_API_PATH != "api.zulip.com",
         'email_integration_enabled': settings.EMAIL_GATEWAY_BOT != "",
         'email_gateway_example':     settings.EMAIL_GATEWAY_EXAMPLE,