All: Added support for hierarchical/nested tags (#2572)

The implementation uses / symbol as a nesting separator. I.e. tag/subtag is a nested tag, where tag is the parent tag and subtag is its child. Creating a tag named tag/subtag/subsubtag creates three tags, one for each level. The tags are associated using parent_id field.

In the app, viewing notes with a tag will also show all notes that are associated with any of the tag's descendant tags (same for the note count). Deleting a tag will also delete all its descendant tags.

In the desktop app the tags are shown nested just like the notebooks.

ReactNativeClient/root.js

@@ -536,6 +536,11 @@ async function initialize(dispatch) {
 			ids: Setting.value('collapsedFolderIds'),
 		});
 
+		dispatch({
+			type: 'TAG_SET_COLLAPSED_ALL',
+			ids: Setting.value('collapsedTagIds'),
+		});
+
 		if (!folder) {
 			dispatch(DEFAULT_ROUTE);
 		} else {