Implement file browser panel resizer

Summary

This is part of a bigger effort to implement a file tree browser (FTB) in the repository.

As part of this issue we should implement file browser panel resizer (the same resizer we user on MR diffs page). This will allow users to resize the repository FTB and persist the user preference in the browser.

Notes

The MR diffs panel resizer stores the user-defined panel size in a cookie/local storage, we should consider using a unique identifier for the stored state for the repository FTB because users might prefer a different panel size on the repository FTB vs MR FTB (to be confirmed with UX)