XWIKI-11760: Convert the WatchList plugin to a component

- Using document initializers for both the 2 classes and the 3 scheduler jobs. Job class and scheduler jobs are only initialized on the main wiki, as before.
- Removed the need to pass the XWiki context to all the methods.
- Added CLIRR exclude for all watchlist classes.
- The code is all in internal packages for now.
- Added a script service to replace the old PluginApi.
- Still using strings instead of references.
- Still throwing XWikiException.
- UI needs fixing to use the script service instead of the plugin API.