Fix checks for serving AMP response and improve AMP compatibility

[westonruter]

Jetpack stats are currently broken for sites running 1.0 of the AMP plugin. See ampproject/amp-wp#1722 and https://wordpress.org/support/topic/v1-0-has-broken-jetpack-stats-tracking/

The problem is that the plugin was not updated after ampproject/amp-wp#1148 (comment) was done during the AMP plugin's beta releases.

To fix, Jetpack needs to wait until after parse_query action has happened in order to determine whether an AMP response is being served.

This PR also:

• Eliminates the omission of the Milestone widget in AMP; instead the widget's output is modified to make it AMP compatible.
• Jetpack stats in the admin bar are made AMP compatible.

Closes #9588.
See also #9730.

/cc @gravityrail

[jetpackbot]

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[gravityrail]

I get a lot of errors like is_amp_endpoint was called incorrectly, caused by some of these hooked functions which are run during init, before parse_query.

Culprits:

invoked directly in likes.php line 272
filter jp_carousel_maybe_disable invoked in jetpack-carousel.php line 102
probably more...

[gravityrail]

I made a modified version of this PR in #10945