Some compatibility fixes

This is three commits to improved compatibility and operation. These are low risk changes, since they only change alpha features (fdroid verify and Binaries:), and otherwise just print warnings.

• change the text field char limits to match other app stores, so that those texts can be easily reused across app stores.
• use APKs as a time source for offline signing machines, print warning if clock is out of sync
• remove wget as dependency, and instead use the Python library requests, which should be used throughout fdroidserver