e2e tests should run periodically and generate failure emails

[audiodude]

The mwoffliner project has many e2e tests that run in CI and test that the project can actually properly process various live wikis and their data. This is great!

In fact, many of the unit tests themselves are actually e2e tests (see #2057)

However, since the external data itself could cause the tests to fail, and we expect this to happen fairly frequently, it would be useful to have the e2e tests run periodically, perhaps once per day (configurable). This would allow us to catch regressions early and not be "surprised" when trying to submit code and having e2e tests inexplicably failing.

Not sure if GitHub actions has this capability but it seems likely.

For example, this was a problem when trying to submit #2055, since it was blocked by the unrelated failure in #2056.