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/lib/components/side-menu-content.js

@@ -402,6 +402,7 @@ const SideMenuContent = connect(state => {
 		// Don't do the opacity animation as it means re-rendering the list multiple times
 		// opacity: state.sideMenuOpenPercent,
 		collapsedFolderIds: state.collapsedFolderIds,
+		collapsedTagIds: state.collapsedTagIds,
 		decryptionWorker: state.decryptionWorker,
 		resourceFetcher: state.resourceFetcher,
 	};