[web] Use signedIdentityKeysBlobSelector in SIWELoginForm

Summary:
Now that we have signedIdentityKeysBlobSelector, we can use it in TraditionalLoginForm (previous diff) and SIWELoginForm to simplify things.

Depends on D7047

Test Plan:

1. Try to "SIWE" from web.

2. Set breakpoint in siweAuthResponder and make sure signedIdentityKeysBlob comes through and is validated as expected.

3. Check the cookies table to see if signed_identity_keys column is populated correctly.

Reviewers: ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7048