Site-wide plugins can be enabled/disabled from the context's plugin grid

[NateWr]

Describe the bug
The Usage Statistics plugin is a site-wide plugin, however, it can be enabled and disabled from the context's plugin grid. This means that a journal manager could disable the plugin even though it should only be possible to do this from the admin area.

The settings form can be saved with different settings for each context, so that needs to be accessible to managers. But it should not be possible to disable this plugin except from the admin area.

To Reproduce
Steps to reproduce the behavior:

1. Go to Settings > Website > Plugins.
2. Click to active/deactivate the Usage Statistics plugin.
3. See the enabled setting in plugin_settings has been changed.

What application are you using?
3.3 pre-release (master)

[NateWr]

The following PR splits the settings management of the usage stats plugin at site/context level. At site level all settings are available and the admin can enable/disable the plugin. At the context level it can not be enabled/disabled, and only the bar chart settings can be configured.

cc @bozana, @ctgraham is that ok?

PR:
pkp/usageStats#38

Tests:
pkp/ojs#2918

[NateWr]

Merged