[Feature]: Synchronize subscription requests across devices

Precondition

[X] I understood that this template is only for feature requests and not for bug reports

Cross-checked related issues?

[X] I have cross-checked this overview https://github.com/monal-im/Monal/issues/322 as well as filtered for related labels https://github.com/monal-im/Monal/labels

Describe your feature

Currently if I have multiple instances of Monal (eg desktop and phone), if I get an incoming subscription request that I want to block, I have to handle them on both devices. Instead, if I react on one instance I want the other to reflect that change.

Eg, if I decline a subscription request, a client receives a stanza like the following and it should handle it by removing the corresponding entry from the list of incoming requests (and send an iq-result accordingly).

<iq xmlns='jabber:client' to='icke@example.org/eli' from='icke@example.org' type='set' id='push6302782379657504161'>
    <query xmlns='jabber:iq:roster' ver='173685c48e2d68315a1e34d4d6d997f25f53a223'>
        <item subscription='remove' jid='spammer@otherserver.com'/>
    </query>
</iq>

<iq type='result' id='push6302782379657504161'/>

PS: This seems to work if I accept the request.

monal-im / Monal