Todos for epics

What does this MR do?

It adds todos for epics. A new todo is created:

• when a user is mentioned (incl. direct mention) on an epic (create, update) or in a discussion
• when a user marks todo in epic sidebar

It removes a todo

• when an epic is re-labled
• when a user marks todo as done in a sidebar

It also adds functionality to filter by group and by epic type

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5481 (closed)