Show a notice for incompatible plugins on the Plugins screen

[SergeyBiryukov]

Background: #43986, #43992.

In a discussion with @TacoVerdo we noticed that while a plugin incompatible with the installed WordPress or PHP version cannot be activated (as expected), the user experience could be improved.

Currently, incompatible plugins cannot be installed from the Plugin Directory as of [43436] / #43986, but if you happen to install such a plugin manually, there is nothing on the Plugins screen that would show its compatibility status. Clicking on the Activate link would just display a wp_die() screen with an error message.

If a plugin does not work with the installed WordPress or PHP version, the Activate button should be disabled, and an appropriate notice should be displayed with a link to the documentation.

This is a follow-up to [43436] / #43986, [44978] / #43992, and similar changes for themes in #48491 and #48507.

Attached is a patch that handles this for the Plugins screen and the Add Plugins screen (disabling the Activate button for incompatible plugins that are already installed).

[SergeyBiryukov]

53990.diff​ includes a new .incompatible CSS class for plugin rows to make sure the notice styling matches the one we have for plugin updates, see 53990.plugins-screen-after.png​.

53990.alt.diff​ just reuses the existing .update CSS class for that, which may or may not be cleaner. I don't have a strong preference yet at this point :)

[SergeyBiryukov]

53990.2.diff​ makes the icon consistent with the one for "There is a new version of ... available, but it doesn't work with your version of PHP" message.

[SergeyBiryukov]

53990.3.diff​ also disables the Network Activate button on network admin Plugins screen.

[SergeyBiryukov]

In 51678:

Plugins: Display a message on Plugins list table if a plugin requires a higher version of PHP or WordPress.

Installation and activation of incompatible plugins was previously disallowed in [43436] and [44978], but if such a plugin was installed manually, there was nothing on the Plugins screen that would show its compatibility status.

Showing an appropriate notice with a documentation link makes the UI more consistent and improves user experience.

Follow-up to [43436], [44937], [44939], [44978], [45043], [45165], [45546], [47573], [47816], [47819], [48172], [48636], [48637], [48638], [48640], [48652], [48653], [48654], [48660].

Props TacoVerdo, SergeyBiryukov.
Fixes #53990.