Make avatar icon size same across the login methods

[hanbyul-here]

• Currently, we set up the icon size for 18px when user logged in with Github, 24px when user logged in with email. That was because (if I remember correctly) we were using default avatar which has a lot of negative space in it, making itself look smaller. Now that we can fetch the avatar image with e-mail address, (but we still don't know it would work or not, so we have to send the fallback image too) it maybe time to make those icons size same.