Enable maintenance mode when a plugin auto-updates

[desrosj]

Currently, when an active plugin is auto-updating, it remains active and the site does not enter maintenance mode.

This can cause a few issues:

• If a user loads the Plugins screen in the admin, the plugin could deactivate if an error is encountered mid upgrade.
• If a user loads the front end of the website and an error is encountered, the WSOD protection will be triggered sending a "false alarm" email to the site admin.

This is a spin-off of #48850 to tackle this in a smaller chunk.

Also potentially related: #34676.

[desrosj]

49400.diff​ uses the same approach that is used in the Theme_Upgrader class, hooking into the pre-install/post-install filters.

[SergeyBiryukov]

The documentation fixes in 49400.1.patch​ look good at a glance, but I'd like to make sure they're consistent with class-theme-upgrader.php.

Let's get 49400.diff​ in for now, and improve the docs after Beta 1.

[desrosj]

In 47275:

Upgrade/Install: Enable maintenance mode when plugins are auto-updated.

When an attempt is made to update an active plugin automatically, there is the potential currently for two negative scenarios:

• The plugin can be deactivated if the Plugins admin screen is loaded when the plugin update is incomplete, causing a PHP error.
• The WSOD protection could be triggered, sending a false alarm email to the site administrator.

By enabling maintenance mode before an active plugin update is attempted, these scenarios can be avoided.

This change implements the same approach as the Theme_Upgrader class of using the upgrader_pre_install and upgrader_post_install hooks to toggle maintenance mode.

Props desrosj, SergeyBiryukov.
Fixes #49400.

[desrosj]

Reopening to improve the inline documentation during beta.