Restrict loading plugins files during NO_PLUGINS

[dvz]

Avoid loading plugin files in the Admin CP's Plugins module when the Disable All Plugins setting (no_plugins) or the NO_PLUGINS constant are set.

While MyBB supports disabling plugins temporarily, the ACP plugin management functions still attempt to load the relevant files to perform relevant actions. In cases of broken plugins, users may thus be required to perform low-level operations to deactivate them (like editing the active plugins cache).

This may result in plugins referenced by codename only when in effect, and may be improved with the planned introduction of non-PHP manifest files for all extensions.