Include images of plugin icons when PMPro is deactivated

[andrewlimaza]

Describe the bug
When Paid Memberships Pro (core) is deactivated, the plugin/Add On icons get lost and default to nothing. See the image below.

Expected behavior
We should fallback to a different resource/path when PMPro is deactivated and include it as a backup in the event PMPro core is deactivated.

There may be a few ways to do this:

• Custom images within the Update manager (Self hosted)
• Offload the image to a URL via the license server JSON. (Last resort)

At the very least we could load the mono-color PMPro logo/icon when PMPro is deactivated.

[ideadude]

Even though we're moving all of our updates in house, we still want to bundle the images for security reasons.