Improve the signup flow to be more solid

[infomiho]

• Research on the best practices regarding verified vs. unverified email signups.
• Research how others implement their signup flows (some initial research: https://www.notion.so/wasp-lang/Email-verification-flow-c2958201f6df4a15a1bdc0e2b0a8a9f5?pvs=4)
• Implement the needed changes

Extra context:

• Injected auth models #1583 (comment)
• https://github.com/wasp-lang/wasp/pull/1583/files#r1439622083
• https://www.notion.so/wasp-lang/RFC-Email-Signup-Flow-11c11a3812084b1eaa00fc42328dcacc

[Martinsos]

One thing I found annoying regarding our current signup flow: if I forget I already have an account with email X and I try to sign up, I get msg "check your email". Which is ok, we don't want to say "email exists", because we would be saying too much that way. But, what does happen is that I keep checking my email for a long time, not knowing if email is coming any moment, or if something went wrong on the sending side, or if I already have an account with that email address (which I might not even think about being possible).

Possible solutions:

1. Change the message to "Check your email (unless account with this email already exists)".
2. If email address already exists, send an email to that address saying "there was an attempted signup with your email into your existing account. If this is not you, ignore this email. If it is you, that means you already have an account."

I like the option (2) better and I think I have already seen it somewhere in action.