UI re-flow bug with project file tree ref / control elements
### Summary
The following discussion from gitlab-ce!22196 should be addressed:
- [ ] @wortschi started a [discussion](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22196#note_114632527): (+1 comment)
> There's a little glitch between breakpoints `576px` and `767px`.
>
> 
### Steps to reproduce
See video in summary.
### What is the current *bug* behavior?
Elements in tree ref flow and doesn't look great.
### What is the expected *correct* behavior?
Elements should flow together and look consistent to the larger breakpoint views.
### Possible fixes
Maybe we can stack the two containers (`tree-ref-container` and the `tree-controls`) on the tablet view so we end up with something like:

We could assign `flex-sm-column flex-md-row` to `nav-block` and then just and have the `.tree-controls` left aligned for this breakpoint.
issue