Mobile Apps: completely remove the edit post link when in app

[jeherve]

Fixes wordpress-mobile/WordPress-iOS#13314

Changes proposed in this Pull Request:

This issue was introduced in #12867.

The original goal of this change was to to hide the Edit Post link
whenever you would access your site via the mobile app's in-app browser.

To hide the Edit Post link, we originally opted to hook into the edit_post_link filter.
While this works, it only changes the string returned for the Edit Post link itself.
It does not account for any "before" or "after" parameters that may be passed
when calling the edit_post_link function; those are not part of the filter:
https://github.com/WordPress/WordPress/blob/001ffe81fbec4438a9f594f330e18103d21fbcd7/wp-includes/link-template.php#L1439

Instead, let's shortcircuit that link earlier, by hooking into the get_edit_post_link filter.
If that filter is set to return an empty string, the edit_post_link function will return early:
https://github.com/WordPress/WordPress/blob/001ffe81fbec4438a9f594f330e18103d21fbcd7/wp-includes/link-template.php#L1420

Matching WordPress.com reference: D38302-code

Testing instructions:

• Start from a Jetpack site using the Twenty Nineteen theme.
• Connect your site to WordPress.com.
• Open your site's frontend in your browser.
• In your browser tools, change your user agent to wordpress/3.1:
  
• Reload the page.
• You should see a little pencil icon at the bottom of your posts, but no "Edit" text or link.
• Now apply this branch.
• The pencil icon should be completely gone.

Proposed changelog entry for your changes:

• Mobile Apps: completely remove the edit post link when in app

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: February 11, 2020.
Scheduled code freeze: February 4, 2020

Generated by 🚫 dangerJS against 36fe271

[designsimply]

LGTM!

I tested using the Twenty Twenty theme with the code change applied and I was able to see that the edit link and the edit icon were both removed from Webviews in WPiOS 14.1.0.0 TestFlight beta on iPhone 6S iOS 13.3.

Before	After

I also verified that I can still see the edit link when viewing the post from Safari directly while logged in as an admin on the site.