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
