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.

[johnbillion]

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.

[johnbillion]

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.

[johnbillion]

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.

[flixos90]

All of these are part of a larger goal.

[johnbillion]

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

[jbpaul17]

This is included in the ​https://make.wordpress.org/core/2017/10/15/improvements-for-roles-and-capabilities-in-4-9/ dev note. Thanks @flixos90!