Create sub-todos / todo events

When you have an active todo for a target (typically an issue or an MR), you won't get a new todo for that same event.

However, it would be very handy to know:

1. How many times todo-creating events have been issued on that target. (In general, the higher, the more important.)
2. Which todo was most recently bumped.
3. If a todo of a 'regular' type (mention) was later superseded by an 'important' type (build failed, assigned, etc.).

This means that we could allow sorting todos on updated_at, but we'd also show the todo if a filter on type matched any of the types in the sub-todo / todo events table. I think we could use some UX here, but I understand how the backend would work, at least 🙂 @pedroms and @dbalexandre, what do you think?

This replaces https://gitlab.com/gitlab-org/gitlab-ce/issues/23293.