[lib] Use message specs when creating notification text

Summary: The interface is not perfect: I would like for it to accept a concrete message type, but it would require deeper refactoring so for now I left it as it was.

Test Plan: Flow. It's hard to test notifs on local, unfortunately.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D644