Todos for epics

• Todos support for epics, similar to issues.
• Same UI on todos page and analogous UI on epic page.
• Same way that an epic todo is triggered, as an issue todo.
• Filter by epics on the todos view.
  • Add “Group” dropdown before the “Project” dropdown, with the same functionality but filtering groups the user is a member of.
    • When filtering by a group, the “Project” dropdown should be cleared, and vice-versa.
    • When filtering by a group, we continue to show todos for all object types (issues, merge requests, and epics). Those objects come from the whole tree that stems from the current group: current group, all of its descendent groups, and all of the projects contained in the group and its descendent groups.
  • Add "Epic" to "Type" dropdown.
• Update todos API.