[Settings] Applying Ace editor height, additional tests

- Adding directive `ace-holder` to dynamically set the height on the ace editor div element so that it fills available space within the settings modal content
- Controller `aceLoaded` function updated to check a flag `initialLoad` as for some reason the ui-ace `onLoad` event gets fired twice, ensuring the logic in `aceLoaded` happens once
- settings HTML updated to remove the unnecessary `<form>` element, remove `disable-save` attribute in `<widget-button-toolbar>` element, and add the import of the ace-holder directive
- additional settings tests added