Mobile fixes

[nsafai]

Okay this PR is becoming huge, and I haven't even started the major mobile re-design / css re-factor yet.

I suggest:
(1) we merge all the fixes in here to master as soon as possible as it fixes some critical / security issues and UX issues, including:

• making sure users ACTUALLY know the correct credentials before letting them through (currently if you try to login and refresh you are logged in)
• making error pages nicer [V1] As a user, I'd like to see nicer error pages & when possible, without any re-directs #77
• cards are centered on mobile [V1] Enhance mobile responsiveness for recipe cards #83
• fix styling on signup page Fix signup page #91
• added color palette and styling improvements
• fixed search so it works on pages other than home
• moving route functions to the more intuitive place (for ex 'cart' GET is moved from controllers/dashboard.js to controllers/cart.js
• Fix helper text so it doesn't say "try another search query" on empty /favorites and /cart
• sort recipes by popularity Sort recipes by popularity #44

(2) I'll open up a new PR to make the mobile design even better.

[phylliswong]

Looks good. Merging into develop.