move timeago.js to vendor directory

## What does this MR do?

moves timeago.js into `/vendor/assets/javascripts` for consistency with other external javascript libraries.

## Are there points in the code the reviewer needs to double check?

make sure timeago still works as it should...?  I don't see how this could have broken anything.

## Why was this MR needed?

consistency with our guidelines

## Does this MR meet the acceptance criteria?

- Tests
  - [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?

See discussion in [!6274](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6274#note_18625583)

See merge request !7590