Support XEP-0313: Message Archive Management

[pravi]

Is your feature request related to a problem? Please describe.
Matrix bifrost bridge is using xmpp.js to implement matrix to xmpp bidirectional bridge but it lacks support for MAM matrix-org/matrix-bifrost#64

This means people connecting from xmmp apps to matrix rooms via the bridge will not see any offline messages.

Describe the solution you'd like
If xmpp.js implements mam support that will make it possible to add this support in the matrix bridge.

Additional context
Matrix bifrost bridge allows bidirectional connections between any public matrix or xmpp rooms. Lack of MAM support is a big missing feature in this setup.

[sonnyp]

#773