feat: Automatically remove oldest unpublished relay in order to make space when the user wants to add more; don't allow more than 5 relays overall (#8428)

Unpublished relays aren't shown in the list of relays anymore, and can't
be deleted manually. Therefore, we want to automatically delete them in
order to make space for more relays when the limit (5) is reached and
the user wants to add another relay.

src/sql/migrations.rs

@@ -2459,8 +2459,8 @@ UPDATE msgs SET state=24 WHERE state=18; -- Change OutPreparing to OutFailed.
     inc_and_check(&mut migration_version, 155)?;
     if dbversion < migration_version {
         // last_rcvd_timestamp tracks timestamp of the last received message
-        // on the transport. This is used to remove hidden transports that
-        // were not used to receive messages for some predefined time.
+        // on the transport. This is used to automatically remove hidden transports
+        // that were not used to receive messages for some time.
         //
         // NOTE: ideally we would use `DEFAULT (unixepoch())`,
         // but sqlite forbids non-constant default in ALTER TABLE.