Upgrade for WP 4.9 and remove back-compat for <4.7

[westonruter]

• Conditionally incorporate [WIP] 4.9 Refactoring Test #161 when on 4.9+.
• Make sure features not merged into core are ported into customize-snapshots.js, including changeset title and saving the current previewed URL and expanded panels/sections as meta when saving a changeset, etc.
• Fix autosave requests. They currently fail with a changeset_post_save_failure for edit_others_posts. There is some conflict between setting the caps in \CustomizeSnapshots\Post_Type::extend_changeset_post_type_object() and the logic to in \WP_Customize_Manager::grant_edit_post_capability_for_changeset() when performing an autosave request.
• Update changeset title when user sitting on customizer and schedule time arrived.
• Test and fix how pending state is saved when user does not have publish capability.

Testing:

• Test if able to publish changeset from front end in compat and 4.9
• Test if able to merge changesets in compat and 4.9
• Test if snapshot admin post edit screen is working as expected in compat and 4.9
• Test state query vars are saved properly in compat and 4.9
• Test if able to create multiple changesets in compat and 4.9
• Overall testing

[coveralls]

Coverage increased (+5.4%) to 80.279% when pulling 922a1ad on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.4%) to 80.279% when pulling 081e066 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.357% when pulling 3993bff on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.357% when pulling 3993bff on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.377% when pulling 54059d2 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.377% when pulling 0b5ef52 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.377% when pulling 86722c8 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.5%) to 80.377% when pulling d471abf on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+6.0%) to 80.859% when pulling 868b6b5 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+6.0%) to 80.859% when pulling 18514d5 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+6.007%) to 80.892% when pulling 558ef8f on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling 558ef8f on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling 96b247e on upgrade/4.9 into 3076044 on develop.

[westonruter]

@sayedtaqui anything left here as far as you know?

[mohdsayed]

I think this is it. I will do more testing before you wake up.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling 026839a on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling 026839a on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling da511c4 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling da511c4 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling da511c4 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling da511c4 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling a2a1ee7 on upgrade/4.9 into 3076044 on develop.

[coveralls]

Coverage increased (+5.3%) to 80.211% when pulling 952a754 on upgrade/4.9 into 3076044 on develop.