Create discussion ADR for key sharing and SSSS

Problem:

The users need keys for individual messages once they log in. We need to figure out how/whether to rely on key sharing and/or SSSS.

ACs: tbd

Solution:

• create an ADR for further evaluation

Notes/resources:

Requirements uIDs: Zapuk_132; LB_5; LB_6; LB_7; LB_8; LB_9; LB_10