Improve TODOs performance

What does this MR do?

It tries to improve TODO list performance by making less queries to routes table.

This improved performance from about 10000-11000 ms to about 7000 ms on my local data (performance depends on user authorised projects and their namespaces, count of routes, distribution of todos between users/projects). The difference on production should be even bigger as the query fetching a single route takes about 0.5 ms on my local db while it takes sometimes more then 5 ms on production db (source: http://profiler.gitlap.com/20170316/3b31bcaf-5972-4877-b314-40cd493e0151.txt.gz)

As described in the related issue we used includes(:namespace) and for every single project (for filter dropdown) a route is fetched additionally. This was changed in this MR.

There is still space for improvements and I am working on it - for every single TODO we fetch 2-3 routes (for avatar, for user - these two could be probably combined and for task target).

This part might be a bit more complicated and I suggest creating another MR for it.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#29534 (closed)