Real-time editing of issue and merge request title

Collaboration around this capability is being consolidate on #21473

Dependencies

• This should be considered after these are implemented: https://gitlab.com/gitlab-org/gitlab-ce/issues/25051 + https://gitlab.com/gitlab-org/gitlab-ce/issues/25049

Description

Like google docs, real time collaboration & autosave. Can maybe be done with

• ShareJS
• Canvas
• Hackmd

Scope

• Consider titles only, so that we don't have to worry about more complicated formatting.
• Consider descriptions separately: https://gitlab.com/gitlab-org/gitlab-ce/issues/44162.