…ss, and Largo

largo_fb_user_is_followable used the Facebook Follow Button,
https://developers.facebook.com/docs/archive/docs/plugins/follow-button/
in order to determine whether a user or page was followable. By using
this public button's HTML markup, we were able to make the followability check
without using the Facebook Graph API.

Facebook deprecated that button on February 5, 2018.

To query the Graph API, we need to have an API key.

To require Largo users to acquire an API key, for the sole purpose of checking
whether their Facebook page is followable, seems excessive.

Therefore, since there's no reasonable way to check whether a Facebook
profile is followable, deprecate the functionality by making the
function always return false, and clean out its tests. If WP_DEBUG or LARGO_DEBUG
are set, provide a deprecation notice to the error log.

…e_no_thumb

…img tag, as it's not necessary for this function and appears to be the cause of #1404

For #1503 (comment)

This change:

- removes an empty <p></p>
- removes an empty <p class="whatever"></p>
- removes the image but not the p if it's a <p><img/>foo</p>
- checks that the foo is left behind
- checks that a non-matching image isn't removed

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