Outbound network requests

Hi!

I am moving an existing Grav installation to another private network, that disallows all outbound connections per default.
The site itself is being delivered as fast as usual, but the admin plugin pages take a long time to load (30 seconds) or result in a 504 Gateway Timeout error. This leads me to suspect that the problems lies with some external requests from the admin plugin (e.g. checking for updates). The basic resources for my deployment are exactly the same as for the previous setup.

Is there a list of URLs that need to be whitelisted in order for the plugin to work correctly? I could not find any documentation on this.

EDIT: I tried using the following config settings, which fixed the issue when accessing **pages**:
```
widgets_display:
dashboard-maintenance: false
dashboard-notifications: false
dashboard-feed: false
enable_auto_updates_check: false
notifications:
feed: false
dashboard: false
plugins: false
themes: false
```

No files or tests are named. Start from the admin plugin pages and the listed widget, notification, and auto-update settings; identify which outbound URLs cause the delays or timeouts, then document the URLs and configuration needed for a private network deployment.

Written by the indexing model from the issue text.