[keycloak-dev] Admin console doesn't reload messages at runtime

The admin console doesn't currently reload messages at runtime. This just doesn't work for development as you have to reload the server each time to get new messages. Any caching of messages (or theme resources in general) should be added to ExtendingThemeManager and made available to all uses of a theme. ExtendingThemeManager already has some caching capability and it's configurable so can be disabled during development. Caching of message bundles would actually be much more important to have for login screens than for the admin console. For now I'm removing AdminMessagesLoader and I'll add caching to ExtendingThemeManager soon.