(core) when redirecting, use protocol in APP_HOME_URL if available

Summary:
Currently, Grist behind a reverse proxy will generate many
needless redirects via `http`, and can't be used with only
port 443. This diff centralizes generation of these redirects
and uses the protocol in APP_HOME_URL if it is set.

Test Plan:
manually tested by rebuilding grist-core and
doing a reverse proxy deployment that had no support for
port 80. Prior to this change, there are lots of problems;
after, the site works as expected.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3400

app/server/lib/SamlConfig.ts

@@ -61,6 +61,7 @@ import {expressWrap} from 'app/server/lib/expressWrap';
 import {GristLoginSystem, GristServer} from 'app/server/lib/GristServer';
 import * as log from 'app/server/lib/log';
 import {Permit} from 'app/server/lib/Permit';
+import {getOriginUrl} from 'app/server/lib/requestUtils';
 import {fromCallback} from 'app/server/lib/serverUtils';
 import {Sessions} from 'app/server/lib/Sessions';
 
@@ -156,7 +157,7 @@ export class SamlConfig {
 
     // Starting point for login. It redirects to the IdP, and then to /saml/assert.
     app.get("/saml/login", expressWrap(async (req, res, next) => {
-      res.redirect(await this.getLoginRedirectUrl(req, new URL(req.protocol + "://" + req.get('host'))));
+      res.redirect(await this.getLoginRedirectUrl(req, new URL(getOriginUrl(req))));
     }));
 
     // Assert endpoint for when the login completes as POST.