Block the creation of a conversion plan if there is already one created with same information

[maririos]

For a product there can only be one In progress or Completed conversion plan for management plane and one for data plane.
We should not let the user create another one as this is a one time process. (they can have conversion plans abandoned, which means they can still create new conversion plans).

Instead, when the user clicks submit we should do the validation and if it fails, let the user know they already have either an active conversion plan (we could link to the view Conversion to TypeSpec plans so the user can look at it), or if the conversion plan was completed, let them know that now they should create a release plan instead (with a link to the create release plan view).