[web] Use signedIdentityKeysBlobSelector in TraditionalLoginForm

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

Test Plan:

1. Try to log in from web.

2. Set breakpoint in loginResponder 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/D7047