config_error: Support passing arguments specifying the "go back" link.

Depending on the kind of config error being shown, different "go back"
links may be more appropriate.
We probably hard-coded /login/ for it, because these config errors are
most commonly used for authentication backend config error, where it
makes sense to have /login/ as "go back", because the user most likely
indeed got there from the login page.

However, for remote_billing_bouncer_not_configured, it doesn't make
sense, because the user almost surely is already logged in and got there
by clicking "Plan management" inside the gear menu in the logged in app.

templates/zerver/config_error/container.html

@@ -22,7 +22,7 @@
 
                     <p>After making your changes, remember to restart
                     the Zulip server.</p>
-                    <p><a href=""> Refresh</a> to try again or <a href="/login/">click here</a> to go back to the login page.</p>
+                    <p><a href=""> Refresh</a> to try again or <a href="{{ go_back_to_url }}">go back to {{ go_back_to_url_name }}</a>.</p>
                 </div>
 
             </div>

zerver/views/errors.py

@@ -1,12 +1,22 @@
+from typing import Optional
+
 from django.conf import settings
 from django.http import HttpRequest, HttpResponse
 from django.shortcuts import render
 
 
-def config_error(request: HttpRequest, error_name: str) -> HttpResponse:
+def config_error(
+    request: HttpRequest,
+    error_name: str,
+    *,
+    go_back_to_url: Optional[str] = None,
+    go_back_to_url_name: Optional[str] = None,
+) -> HttpResponse:
     assert "/" not in error_name
     context = {
         "error_name": error_name,
+        "go_back_to_url": go_back_to_url or "/login/",
+        "go_back_to_url_name": go_back_to_url_name or "the login page",
     }
     if settings.DEVELOPMENT:
         context["auth_settings_path"] = "zproject/dev-secrets.conf"

zerver/views/push_notifications.py

@@ -234,4 +234,9 @@ def self_hosting_auth_not_configured(request: HttpRequest) -> HttpResponse:
         # is actually real.
         return render(request, "404.html", status=404)
 
-    return config_error(request, "remote_billing_bouncer_not_configured")
+    return config_error(
+        request,
+        "remote_billing_bouncer_not_configured",
+        go_back_to_url="/",
+        go_back_to_url_name="the app",
+    )