server, ui: set HttpOnly: true and Secure flags on cookies

[dhartunian]

server, ui: session cookie is HttpOnly

This change removes the ability of the DB Console JS code to read
the session cookie from the browser by setting the HttpOnly flag
to true.

This change requires a modification of how the TenantDropdown
components works since it relied on reading the session cookie and
extracting the list of logged-in tenants from it.

Instead of reading the cookie, the component now relies on a server-
side endpoint to "read back" the list of tenants in the session. You
might ask why this is necessary because we could return the list of
valid tenants on /login success and have them available outright.
There is a reason why that's not sufficient.

When you are logged in to multiple tenants with a valid session, you
can switch between them in the dropdown switcher. This switch occurs
without a subsequent call to /login, and will just set the tenant
cookie to the new tenant name and issue a full refresh to the browser
which will re-render everything. In this case, the TenantDropdown
component gets fully re-rendered but no longer has access to the
state it was initialized with containing all the valid tenants. This
necessitates adding an endpoint that the component can use to populate
itself directly whenever it's rendered.

Epic: None
Fixes: CRDB-36034

Release note (security update): DB Console session cookie is now
marked HttpOnly to prevent it from being read by any Javascript
code.

---

server: set Secure on cookies if cluster is secure

Cookie Secure setting is based on disableTLSForHTTP passed down
from the server.

Part of CRDB-36034
Epic: None

Release note (security update): DB Console cookies are marked Secure
for the browser when the cluster is running in secure mode.

[cockroach-teamcity]

This change is 

[stevendanna]

I think we should probably merge this as-is as it represents the code that was merged to release branches.

I still think it would be nice to follow up with a change in which al cookie creation goes through one function where it will be easier to audit in the future.

[dhartunian]

bors r=stevendanna

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)