[tiptap] Mention users

[JammingBen]

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

User Story

As a user, I want to be able to mention other users in the text editor so that I have a quick and easy way to notify them.

User Value

UX

Acceptance Criteria

• typing @ in a new line or after a blank opens a drop menu that lists all the users that have access to the current document (space members & share receivers)
• selecting a user puts it into a temporary list
• saving the document calls the collaboration/notification endpoint on the server with selected users from that list (see packages/web-app-external/src/composables/useCollaboraPostMessages.ts:305)