[web] "Unpickle" primary OLM account to sign keys and get SignedIdentityKeysBlob

Summary:
We unpickle the primary OLM account in order to sign a "concatenation" of the primary and notification keys.

We then bundle up the payload and signature into SignedIdentityKeysBlob which is what will get included in the login request to the keyserver.

Depends on D6883

Test Plan:
Logged identity_keys() of "unpickled" account and it matched the keys from the instantiation of the primary account:

signedIdentityKeysBlob is created as expected:

Reviewers: ashoat, tomek, marcin

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6884