Add sources service and frontend (#6774)

# Description of Changes
Redesign policies backend to treat sources a lot closer to how the
frontend imagined them working (they're persistent now and have an API).
Then connect the portal to the sources when mocks are off to allow for
source creation in the UI. It's not particularly useful to do that right
now because there's no policies UI, but I've tested manually that
sources set up in the UI are usable by policies created via the API.

I had to change the portal so that when mocks are off, it doesn't just
hard crash when attempting to connect to all the backend APIs that don't
exist yet. It'll still log the errors, but just continues on rendering
the UI now.

I also changed all the policies backend APIs to be gated behind a flag
instead of behind the SaaS profile. This is because we haven't yet got
the payment model sorted, but we're going to need this stuff running
self-hosted to be able to test it locally.

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -206,6 +206,11 @@ public class ApplicationProperties {
 
     @Data
     public static class Policies {
+        /**
+         * Master switch for the policy + sources subsystem (the PAYG-metered automation surface).
+         */
+        private boolean enabled = false;
+
         /**
          * Absolute directories that policy folder input sources and output sinks may read from or
          * write to. Empty (the default) disables folder access entirely, so a policy can never be