(core) Improve the look and behavior of /welcome/teams page (also shown for /welcome/start)

Summary:
- Move css module for the login page css to core/, to be reusable in core/ pages.
- Move /welcome/teams implementation to WelcomeSitePicker.ts
- List users for personal sites, as well as team sites.
- Add org param to setSessionActive() API method and end endpoint, to allow
  switching the specified org to another user.
- Add a little safety to getOrgUrl() function.

Test Plan: Added a test case for the new behaviors of the /welcome/teams page.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3914

static/locales/en.client.json

@@ -1030,5 +1030,10 @@
     },
     "GridView": {
         "Click to insert": "Click to insert"
+    },
+    "WelcomeSitePicker": {
+        "Welcome back": "Welcome back",
+        "You can always switch sites using the account menu.": "You can always switch sites using the account menu.",
+        "You have access to the following Grist sites.": "You have access to the following Grist sites."
     }
 }