Backport SmartInterval, PrettyTime, SubbableResource from EE.

## What does this MR do?

Backports infrastructure used for EE-only Timetracking so it can be used and improved upon in CE. 

This doesn't really need review... it was already reviewed and merged in EE. 

There are no side effects or conflicts, just three new classes added:

1. `SubbableResource` -- pubsub for ajax resources
2. `SmartInterval`-- for configurable polling
3. `PrettyTime` -- time parsing and formatting utility methods

- [x] Added for this feature/bug
- [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/985

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/870

cc: @jschatz1

See merge request !7573