plugin XEP-0277: post repeating implementation: Repeating a post as specified at https://xmpp.org/extensions/xep-0277.html#repeat is now implemented. On sending, one can either use the new `repeat` method (or the bridge `mbRepeat` one from frontends) to repeat any XEP-0277 item, or use the field `repeated` in microblog data's `extra`. This field must mainly contains the `by` field with JID of repeater (the "author*" field being filled with original author metadata), and the `uri` field with link (usually `xmpp:` one) to repeated item. When receiving a repeated item, the same `repeated` field is filled. rel 370

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