MintBadge form validation

Currently the [form validation for the Mint Badges](https://github.com/bafnetwork/baf-badges/blob/5dcb954c59c9f438578bdf28611b34dbf8e262e3/pages/mint-badges.tsx#L29) form does not check the accountID's to make sure they're real NEAR accounts.

Tasks use [vestjs](https://vestjs.dev) to:
- [ ] validate the account ID's given by the form to make sure they're valid NEAR account IDs and they exist
- [ ] validate the optional url given to make sure it's a valid URL

Start in pages/mint-badges.tsx at the linked form-validation code and read the VestJS documentation for the validation setup. Confirm how to check the account IDs for valid, existing NEAR accounts and how to handle the optional URL; the work is done when invalid account IDs, nonexistent accounts, and invalid URLs are rejected while valid submissions pass.

Written by the indexing model from the issue text.