Bug Description

The checkbox introduced in #525 does not show up when using Multisite. This is because the entire settings section (group media, section uploads) is not rendered for multisites. We have to fix this, which I think could be done in two alternative ways:

• Either we keep control about this per site in the multisite, thus just conditionally render it in another settings section on the same page when using a multisite (e.g. the default section).
• Or we only grant control about this at the network level, thus add the checkbox to the Network Admin's settings screen.

Steps to reproduce

1. Use a Multisite.
2. Ensure the WebP Uploads module is active.
3. Go to Settings > Media, and notice that the checkbox to opt in to WebP and JPEG generation is not there.

Screenshots

Settings > Media when using a multisite