Weak Bithdate Validation Implemented on Sign Up

The Birthdate Field on the KhanAcademy's [Sign Up](https://www.khanacademy.org/signup) page for new users has the year range from 2017 to 1897. {F210177} However, while signing up for a new account, I was able to set the year to 1033 by manipulating the data being sent to the server and the account was successfully created. I can also confirm the year by checking the settings page of my account. Please refer to the screenshots for information. {F210178} {F210179} {F210180} {F210181} I was also able to create a Child account with the same bug. Attaching the screenshot where the system says "You don't have an email address connected to this Khan Academy account. Since you're **_1017 years old_**, you can connect an email address to your account, which will unlink it from your parent's account.". {F210182} {F210183} Suggestion: Proper validation should be implemented on Birthdate field so that the user is not able to set any year other than what is being displayed on the dropdown.