added icon to toggle sidebar

[rahulrana701]

Fixes #2953

Changes:

I have verified that this pull request:

• [X] has no linting errors (npm run lint)
• [X] has no test errors (npm run test)
• [X] is from a uniquely-named feature branch and is up to date with the develop branch.
• [X] is descriptively named and links to an issue number, i.e. `Fi

  xes #2953 `

[rahulrana701]

@lindapaiste I have added an icon to toggle the sidebar.

[raclim]

Thanks for taking the time to work on this!

I definitely think this improvement is needed, but feel that we want to approach new design changes cautiously and intentionally and solidify on one before merging them in. I'm not sure if implementing this arrow is the best solution for this, especially because it obscures the "New File" text and might feel a bit crowded next to the "plus" icon. In light of wanting to approach implementing new designs carefully and some bug fixes/refactoring we want to prioritize at this moment, I don't think we're planning to get this in in the near future. I'm sorry that we can't get this in at the moment, but I do think this would be great to revisit once a design is decided on in the affiliated issue!