Implement form validation for relevant post types

[ori-near]

• [ ] For idea, solution, sponsorship and attestation, Post Title and Description Cannot be both Empty
• [x] For sponsorship and solution, Requested amount cannot be empty and must be positive number
• [ ] For solution, if request sponsor is provided, must be valid account id
• [ ] For sponsorship, supervisor must be valid account id
• [ ] For comment, description cannot be empty

Question: should we also restrict sponsorship supervisor to be the same account who created the sponsorship post?