[web] Rename SIWE component to SIWELoginForm

Summary: Clearer name and also parallel naming structure to TraditionalLoginForm and also distinguishes from SIWE component in landing.

Test Plan: NA, flow + things work as expected + searched codebase for usages of the web SIWE component and this was the only one.

Reviewers: ashoat, tomek, rohan, ginsu

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6502