Adding a topic collection to itself

**Acceptance criteria**
- [ ] User is able to add a topic collection to a topic collection, but not to itself
- [ ] When editing a topic collection, an author should only see other topic collections, and not the current topic collection being edited

**Describe the bug**
Topic collections can be nested under topic collections, which is cool per our IA rules (#2575).
What's not cool: currently, it's possible to add a topic collection to itself. It shouldn't be.

**URL**
http://joplin-staging.herokuapp.com/admin/pages/150/edit/

**To Reproduce**

1. Go to Joplin
2. Edit an existing topic collection or create a new topic collection
3. Scroll down to 'Select a topic collection'
4. Click "Add topic collections..." button
5. Click "Choose topic collection..." to open the explorer modal

**Current behavior**
You'll see the topic collection you're currently on as an option—and you can choose it!

**Expected behavior**
The current TC you're on shouldn't be on the list, or somehow indicate that it's not an option and a user doesn't have the ability to select it.

**Screenshots**

**Additional context**

Start at the topic collection edit flow shown by the admin URL and reproduce the explorer modal steps in the issue. Done means a topic collection can contain other topic collections, but the collection currently being edited is absent from the selectable list and cannot be added to itself.

Written by the indexing model from the issue text.