fix: removed unique username and email validations, added other validations to backend

[nicolasauler]

To make the application secure, we need to avoid user enumeration, this PR starts the effort, we need:

• Consistent return when user/email exists and don't exist in sign up (implemented in this PR)
• Consistent timings for the above routes sec: make timings consistent when user enumerations would be possible through timing attacks #62
• Clear user communication (sending forgot password email when user exists and doing the appropriate logic: https://cheatsheetseries.owasp.org/cheatsheets/Forgot_Password_Cheat_Sheet.html)
• Forgot password functionality feat: forgot password #33
• Rate limiting in these endpoints sec: add rate limiting to public endpoints and more vulnerable endpoints #61
• Captcha feat: add captcha #20

This PR also adds validations that occurred only when using the application via front, to the backend as well.
And as a bonus, the PR moves script loading to the appropriate html pages, instead of loading them all at the base template.
And also removes unnecessary db accesses when user is already in session.

Closes #59
Closes #57

[nicolasauler]

QA: email was sent and sign up flow has been checked.

[nicolasauler]

QA: password verification at backend has been checked by sending requests directly with curl