… handling

* Add admin settings for picking between disabled, paired, and native theme support. See #1196.
* Add checkbox for automatically allowing tree shaking.
* Add checkbox for automatically sanitizing all validation errors (including tree shaking).
* Make explicitly clear that unaccepted validation errors will block rendering on AMP
* Prevent serving dirty AMP by forcibly sanitizing all validation errors when amp_is_canonical(). See #1192.
* When validation errors are automatically sanitized, ensure the terms' term_group is updated when re-checking.
* Update AMP_Options_Manager::get_options() and to return default values.

…dd/admin-theme-support-options

…tion errors

… (and why)

* Indicate the count of validation errors that are blocking AMP from being available.
* Allow passing URL string in addition to amp_invalid_url post to AMP_Invalid_URL_Post_Type::get_invalid_url_validation_errors().

…ence

…ut which are still forcibly sanitized

* Fix PHP 5.3 error
* Remove automatically setting amp_validation_error term_group based on forced status.
* Let preview URL include development=1 flag.

…s allowed

* Update post editor warnings to reflect when validation errors are forcibly sanitized but
not explicitly accepted.
* Allow tree shaking setting to be shown when in native mode.
* Show notice when selecting native mode that sanitization is required.
* Hide the preview button from the amp_invalid_url post if sanitization is required.

…te link

Discontinue automatically-deleting an amp_invalid_url post when all errors are resolved.