Introduce singular capabilities for activating and deactivating individual plugins

[johnbillion]

As we did in #35614 for taxonomy terms, singular capabilities should be introduced for managing individual plugins.

This would allow fine-grained cap checks such as current_user_can( 'activate_plugin', $plugin_file ).

This ticket covers activating and deactivating plugins.

I decided to split this up a bit to avoid huge patches. This ticket now only addresses activating and deactivating plugins, and we can open other tickets for plugin editing, deletion, installation, etc, as necessary.

Patch attached.

38652.activate-deactivate.diff​ covers the following situations:

• Activate/Deactivate links on the Plugins listing screen.
• Actual activation and deactivation cap check, including bulk activation and deactivation.
• The 'Activate' link shown after a shiny plugin installation.
• The 'Activate Plugin' link shown after a non-shiny (ie. fallback) plugin update.
• Plugin error scraping.

It does not cover network activation/deactivation. This can be handled separately.

For clarity, the newly introduced activate_plugin, deactivate_plugin, and deactivate_plugins meta capabilities all map to the existing activate_plugins primitive capability, so there is of course no change in existing behaviour.

All of these are part of a larger goal.

In 41290:

Plugins: Introduce singular capabilities for activating and deactivating individual plugins.

This introduces the following meta capabilities:

• activate_plugin
• deactivate_plugin
• deactivate_plugins

The singular activate_plugin and deactivate_plugin capabilities are used along with the corresponding plugin name when
determining whether or not a user can activate or deactivate an individual plugin.

The plural deactivate_plugins capability is used in place of the existing activate_plugins capability when determining
whether a user can deactivate plugins.

Each of these new meta capabilities map to the existing activate_plugins primitive capability, which means there is no
change in existing behaviour, but plugins can now filter the capabilities required to activate and deactivate individual
plugins.

Fixes #38652