[web] Pickle OLMAccounts and add to Redux CryptoStore

Summary:
We pickle the OLMAccounts and add them to Redux CryptoStore so the olm.Account can be "unpickled" later for eg signing a message.

Depends on D6882

Test Plan:
Pickled accounts appear in Redux CryptoStore as expected:

(Yes, there are four separate actions being dispatched at the moment. Once things are working end-to-end we can refactor/consolidate as needed.)

"Unpickling" will be tested in subsequent diffs.

Reviewers: ashoat, tomek, marcin

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6883