Add Login/Logout functionality to mobile layout

[ghalestrilo]

Requires #1528

This PR adds an option to the <MobileIDEView /> Dropdown menu, allowing the user to log in and out successfully. The <Dropdown /> component has also been changed.

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[catarak]

Yay this is so exciting! Two things I noticed:

• 

The dropdown menu items are not centered instead of left-aligned.

• 

This nav should probably match the nav styling of what's on mobile! Thought it also makes me wonder... there's no p5.js logo on the mobile design and maybe it should be somewhere?

[ghalestrilo]

I aligned the dropdown items! As discussed, I think it makes more sense to place the p5 logo on a new branch

[catarak]

Possibly makes sense to put this in another PR (I'm going to approve this and let you make that decision), but other thoughts: - When you click "back to editor" it takes you to the desktop view - When the translation is enabled the Nav gets all smooshed

[ghalestrilo]

Ok! I think the p5 logo makes sense to go in another branch, and about the redirect: #1564 fixes it by making the mobile view render on the index endpoint, when appropriate. I'll work on the CSS to fix the header and merge this

[ghalestrilo]

Updated the header on both <SignupView /> and <LoginView /> on small resolutions. Some notes here:

1. A workaround was necessary to implement width detection. This will be fixed in Feature/switch mobile desktop #1564 , which implements more robust responsiveness.
2. Sign Up and Login In options were hidden from the header (on mobile) because they're already available on-screen
3. Some margins and paddings were reduced to fit content better on small resolutions.

I'll merge this PR because others depend on it, but I we should test this on different resolutions before deploying, and open issues if changes are required.

Edit: Needs a new review before merging ✨ @catarak please review it again when you can 😬

[ghalestrilo]

Just made a final update: right-aligning the language dropdown so it's visible on mobile

[catarak]

Looks great to me 🌈