New beta editor for the mobile app🔗

The latest Android pre-release 24 features an improved beta editor, which I hope could become a replacement for the very basic editor we have at the moment.

It's still experimental because it's based on the equally experimental CodeMirror 6, however for simple editing tasks it seems to work fine. At the moment the improvements are:

- Syntax highlighting for various tags such as bold, italic and headings.

- List continuation for ordered and unordered lists (I didn't try checklists but I assume it doesn't work)

- Improved undo/redo

- Maybe better handling of large documents? CodeMirror 6 has a demo that loads a document with millions of lines, so maybe that will solve the performance issues that some users were having

If everything works well, later on we should be able to add things like a toolbar, spellchecking and other features that are impossible with the current editor.

If you find any bug, feel free to report here. Also make sure you backup your notes regularly in case there's an issue!