plugin XEP-0277: better resilience to broken items: - if `author` element can't be found, `publisher` attribute, then `IQ`'s `from` attributes are used as fallback to find author jid - fix categories (tags) parsing if `author` element is not found - remove items which have failed parsing from `mbGet` results (instead of using `None`).

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)