New editor

[RobbinBaauw]

This combines many previous issues. We want to build a new editor with tiptap. It must have the following features:

• Markdown support
• WYSIWYG support
• Realtime selection / viewing of online people
• Post header support
• Realtime editing / saving
• Katex / code examples support

Markdown / WYSIWYG

The current editor has hacky support for Markdown. This should be fixed, markdown should be just as important as WYSIWYG

Realtime selection / viewing of online people

You should see who is editing and who selects what

Post header support

• Each <h1> gets a + button next to it to collapse and collide it (to make it cleaner to read)
• Upon selecting <h1> through <h3>, you get a "copy link" button, which copies the link with the fragment
• There should be an overview of all the headers so you can easily scroll to them

Realtime editing /saving

The current realtime editing and saving works, but saving is slow and hacky. Realtime editing is fine but a well tested solution would be better. Automerge is a good contender here

Katex / code examples

Very important, support for this must stay