Ensure Briefcase can run while offline

As long as you have previously downloaded templates, support packages etc, it should be possible to run Briefcase while you have no internet connection.

At present, being offline raises loud errors.

There may be other online checks that need to be gated.

Two changes are required:
* There should be an `--offline` command line argument that will silence (or at least radically quieten down) warnings related to being offline
* Any request to access the network should be able to fallback to previously downloaded files.

The `NetworkFailure` exception may provide a useful way to identify and catch these cases.

Start by tracing Briefcase's network-access paths and the NetworkFailure exception mentioned in the issue. Identify where the CLI can accept an --offline option and where previously downloaded files are selected. Done means offline runs avoid loud network errors, use cached files when available, and cover the relevant online checks.

Written by the indexing model from the issue text.