Feature: conversation contexts

[chaoflow]

• I have searched open and closed issues for duplicates

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It would be great if I could organize my contacts in such a way that I can select only specific subsets to be active in order to establish different conversations contexts (e.g. work, specific client, private, ...) Conversations and notifications would be displayed only for and searching happen only within activated contexts.

The simple use case would be toggling between work and private contacts.

In a more complex use case I would like to enable contacts from a specific client and my colleagues but not from any other client or private.

One solution would be to label contacts and select labels to be active.

relates #430