(core) add a sandbox check to admin panel, and start reconciling boot and admin pages

Summary:
This adds a basic sandbox check to the admin panel. It also makes
the "probes" used in the boot page available from the admin panel,
though they are not yet displayed. The sandbox check is built as
a probe.

In the interests of time, a lot of steps had to be deferred:
 * Reconcile fully the admin panel and boot page. Specifically, the
   admin panel should be equally robust to common configuration problems.
 * Add tests for the sandbox check.
 * Generalize to multi-server setups. The read-out will not yet be useful
   for setups where doc workers and home servers are configured
   separately.

Test Plan: Added new test

Reviewers: jarek, georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4241

app/common/BootProbe.ts

@@ -1,9 +1,11 @@
+import { SandboxInfo } from 'app/common/SandboxInfo';
 
 export type BootProbeIds =
     'boot-page' |
     'health-check' |
     'reachable' |
     'host-header' |
+    'sandboxing' |
     'system-user'
 ;
 
@@ -20,3 +22,4 @@ export interface BootProbeInfo {
   name: string;
 }
 
+export type SandboxingBootProbeDetails = SandboxInfo;

app/common/SandboxInfo.ts

@@ -0,0 +1,9 @@
+export interface SandboxInfo {
+  flavor: string;       // the type of sandbox in use (gvisor, unsandboxed, etc)
+  functional: boolean;  // whether the sandbox can run code
+  effective: boolean;   // whether the sandbox is actually giving protection
+  configured: boolean;  // whether a sandbox type has been specified
+  // if sandbox fails to run, this records the last step that worked
+  lastSuccessfulStep: 'none' | 'create' | 'use' | 'all';
+  error?: string;       // if sandbox fails, this stores an error
+}

app/common/gristUrls.ts

@@ -87,6 +87,7 @@ export const commonUrls = {
   helpCalendarWidget: "https://support.getgrist.com/widget-calendar",
   helpLinkKeys: "https://support.getgrist.com/examples/2021-04-link-keys",
   helpFilteringReferenceChoices: "https://support.getgrist.com/col-refs/#filtering-reference-choices-in-dropdown",
+  helpSandboxing: "https://support.getgrist.com/self-managed/#how-do-i-sandbox-documents",
   freeCoachingCall: getFreeCoachingCallUrl(),
   contactSupport: getContactSupportUrl(),
   plans: "https://www.getgrist.com/pricing",