pgAdmin 4: Non-administrators can share a server with every user

In pgAdmin 4 server mode only administrators are meant to share a server with all users; the server dialog shows the Shared switch only to them. The create and update endpoints of the server API, however, stored the "shared" flag and shared username they were sent, so any user able to register a server could share one with every other user. The servers.json import had a similar gap: a non-administrator's "Shared" entries were removed only after validation, so a file that also held an invalid entry could create a shared server. A shared server carries no credentials, so this gives no direct access. It lets a user plant a server, named to look trusted, in every other user's tree and pointing at a host the user controls; anyone who then connects to it presents their own database password to that host. The fix refuses a non-administrator's request that would share a server with HTTP 403, prunes a non-administrator's shared entries from a servers.json import before validating it, and warns at startup about servers shared earlier by non-administrators. This issue affects pgAdmin 4: before 9.19.