Add mobile blocker page

[as-dr]

Closes #344

[Schwartz10]

@listenaddress a couple things left in here for you:

1. add logic for isMobileOrTablet - i already found some code for detecting mobile, just need to find a good one for tablets.
2. add tests for isMobileOrTablet
3. Add tests for the useDesktopBrowser hook, you can stub 2 things here:
   a. you can stub the isMobileOrTablet as a jest mocked function, so it returns true
   b. you can stub next/router module so that you can assert that the replace function was called, passing /error/use-desktop-browser. I do something similar here: https://github.com/openworklabs/filecoin-web-wallet/blob/add/mobileblocker/lib/check-network.test.js#L10-L11 but you might need to use a different strategy, that uses spyOn for this (something like How to mock useRouter? vercel/next.js#7479 (comment))
4. snapshot testing for the error pages could go in this PR, or we could open that in a separate follow-on PR