Comp 282 add observer role

This MR adds the observer role. The main difference is that observers cannot be assigned to an NREN (the assignment is ignored) - and observers have read-only access to all surveys.

The landing page for observers has a table which lists links to the latest survey to each NREN, instead of directly navigating to a specific survey.

compendium_v2/routes/response.py

@@ -90,7 +90,7 @@ def check_access_nren_read(user: User, nren: str) -> bool:
 
 
 def check_access_nren_write(user: User, nren: str) -> bool:
-    if not check_access_nren_read(current_user, nren):
+    if not check_access_nren_read(user, nren):
         return False
     if user.is_observer:
         # observers can't edit their own nrens either!