[web] Pull signedIdentityKeysBlobSelector out of getSignedIdentityKeysBlobSelector

Summary:
We extract signedIdentityKeysBlobSelector out from getSignedIdentityKeysBlobSelector in this refactor so we can reuse it in TraditionalLoginForm() and SIWELoginForm().

Depends on D7036

Test Plan: Will be tested in subsequent diffs where signedIdentityKeysBlobSelector is consumed from TraditionalLoginForm() and SIWELoginForm().

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7037