DO instance: improve URL validation

In the BE, the url field of a DO instance is a URLField object, and Django automatically checks for the correctness of the URL. However, it requires the url to have the protocol prefix (https), which isn't very user friendly, this could be improved by removing this part of the check

Start in the backend code for the DO instance and inspect the url field's URLField validation. Confirm the current protocol requirement and adjust the validation so a user-entered URL without https is accepted while malformed URLs remain rejected; done means the intended URL forms validate successfully.

Written by the indexing model from the issue text.