[REFACTOR] Use constance variables in email template and add login URL

breccia_mapper/settings.py

@@ -348,6 +348,9 @@ CONSTANCE_CONFIG = {
     'RELATIONSHIP_FORM_HELP': (
         '',
         'Help text to display at the top of relationship forms.'),
+    'DEPLOYMENT_URL': (
+      'http://localhost',
+      'URL at which this mapper tool is accessible'),
     'PARENT_PROJECT_NAME': (
       '',
       'Parent project name'),
@@ -441,6 +444,9 @@ CONSTANCE_CONFIG_FIELDSETS = {
         'ORGANISATION_LIST_HELP',
         'RELATIONSHIP_FORM_HELP',
     ),
+    'Deployment': (
+        'DEPLOYMENT_URL',
+    ),
 }  # yapf: disable
 
 CONSTANCE_BACKEND = 'constance.backends.database.DatabaseBackend'