Co-authored-by: Weston Ruter <westonruter@google.com>

This reverts commit 8cfa8be.

- Use `amp-first` query arg to force validation in Standard mode
- Use `omit_stylesheet` and `cache` flags

- Replace `useState`s with a single `useReducer`
- Colocate site-scan-related sub-components

…ings screen

Co-authored-by: Weston Ruter <westonruter@google.com>

- Store validation errors directly inside the `scannableUrl` array and derive theme and plugin issues, and staleness flag from that array.
- Store HTTP request errors in the state so that it's possible to find out if the entire scan was unsuccessful (every `scannableUrl` has an `error` set).
- Handle `isFailed` status in the UI.

…mode` spec

…e-scan

* add/validate-store-url-query-var:
  Improve tests for send_validate_response
  Add test for maybe_send_cached_validate_response
  Test omission of stylesheets in send_validate_response
  Replace redundante staleness with revalidated prop
  Move logic into maybe_send_cached_validate_response method
  Introduce args for validate requests to cache, serve cached, and omit stylesheets
  Add test coverage for AMP-first override requests
  Improve send_validate_response logic and add test coverage
  Move validate logic to validation manager
  Revert "Add amp_available_override query param to force AMP to be available"
  Adopt the Web Stories approach of selectively enabling Standard mode
  WIP: Try facilitating selective AMP-first URLs when in paired mode
  Add amp_available_override query param to force AMP to be available
  Remove needless pretty-printing of validation response
  Use constant for amp_store query param
  fixup! Allow validate requests to also store results
  Make is_validate_request a public method and member var protected
  Allow validate requests to also store results

…cture

…719-site-scan

* 'develop' of github.com:ampproject/amp-wp:
  Remove extraneous action priority

Co-authored-by: Weston Ruter <westonruter@google.com>

It also improves the logic that renders Site Scan info notices.