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/migrations/31.js

@@ -0,0 +1,33 @@
+const Tag = require('lib/models/Tag');
+
+const script = {};
+
+script.exec = async function() {
+	const tags = await Tag.all();
+
+	// In case tags with `/` exist, we want to transform them into nested tags
+	for (let i = 0; i < tags.length; i++) {
+		const tag = Object.assign({}, tags[i]);
+		// Remove any starting sequence of '/'
+		tag.title = tag.title.replace(/^\/*/, '');
+		// Remove any ending sequence of '/'
+		tag.title = tag.title.replace(/\/*$/, '');
+		// Trim any sequence of '/'+ to a single '/'
+		tag.title = tag.title.replace(/\/\/+/g, '/');
+
+		const tag_title = tag.title;
+		let other = await Tag.loadByTitle(tag_title);
+		let count = 1;
+		// In case above trimming creates duplicate tags
+		// then add a counter to the dupes
+		while ((other && other.id != tag.id) && count < 1000) {
+			tag.title = `${tag_title}-${count}`;
+			other = await Tag.loadByTitle(tag.title);
+			count++;
+		}
+
+		await Tag.saveNested(tag, tag.title);
+	}
+};
+
+module.exports = script;