Ensure frontend links and Ajax requests include UUID query param; add previewing of Ajax requests and form submissions

[westonruter]

When previewing a snapshot on the frontend, the admin bar is modified accordingly:

- (Unchanged) The “Customize” link takes you to the Customizer with that snapshot state applied. - The “Inspect Snapshot” link takes you to the edit post screen (formerly submenu item under Customize). - A new “Exit Snapshot Preview” link appears which removes the `customize_snapshot_uuid` query param from the current URL. - All other admin bar links are removed, other than those which already contain the `customize_snapshot_uuid` query param (as in the Airplane Mode toggle shown here), or which are in-page links, such as the query monitor link.

When you are navigating around the site with a snapshot applied, the sessionStorage will store the UUID for this snapshot. While the JS will attempt to inject the customize_snapshot_uuid into all links in the document, if you do navigate away from a URL that has this query param, the JS on the new non-snapshotted page will see that you have a snapshot UUID in sessionStorage but no snapshot in the URL, and it will add an admin bar link to prompt you to resume the snapshot preview:

This PR also ensures that Ajax requests made on the frontend include the customize_snapshot_uuid param so that the responses can be previewed. Additionally, Ajax requests made inside the Customizer preview also now get the Customizer state added so that their responses should be previewable as well.

Lastly, forms that have GET request methods (such as the search form) now are actually usable in the Customizer preview! Likewise, forms on the frontend will persist the current customize_snapshot_uuid in the requests. This fixes a long-standing Trac ticket #20714: Theme customizer: Impossible to preview a search results page. See sub-PR #72.

Fixes #47
Fixes #65

[coveralls]

Coverage decreased (-0.9%) to 86.538% when pulling c1ce097 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+0.1%) to 87.557% when pulling 8bf8be2 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+0.2%) to 87.585% when pulling 0d74731 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+0.3%) to 87.753% when pulling ae9593d on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.6%) to 88.999% when pulling 1100a2e on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.7%) to 89.13% when pulling dd8e944 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.7%) to 89.13% when pulling 32c9c71 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.7%) to 89.103% when pulling e08290a on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.7%) to 89.103% when pulling e08290a on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+0.6%) to 88.013% when pulling 50b04d3 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+1.1%) to 88.536% when pulling f2e5a17 on feature/previewing-improvements into a928d18 on develop.

[coveralls]

Coverage increased (+2.1%) to 89.56% when pulling be2a9cd on feature/previewing-improvements into a928d18 on develop.

[valendesigns]

@westonruter Looks good! 👍

[coveralls]

Coverage increased (+2.1%) to 89.511% when pulling 9d1ad7a on feature/previewing-improvements into a928d18 on develop.