Improvements for product/project template refactoring

[maxceem]

This issue has the final touch which we should do for refactoring product template and project templates to use Form model.

It's a follow up from this PR #290

1. In this PR helper method has been created util.checkModel while in previous code this method has been defined several times:

   • https://github.com/topcoder-platform/tc-project-service/blob/feature/product-templates-refactoring/src/routes/projectTemplates/create.js#L50
   • https://github.com/topcoder-platform/tc-project-service/blob/feature/product-templates-refactoring/src/routes/projectTemplates/update.js#L53
   • https://github.com/topcoder-platform/tc-project-service/blob/feature/product-templates-refactoring/src/routes/projectTemplates/upgrade.js#L55

   So we can reuse a new helper method in all these cases.

2. At the moment we have two ways of defining forms for both ProductTemplates and ProjectTemplates: old way inside the template and new way using Form model.
   So both of fields are optional:

   

   and

   

   For ProductTemplate we should check in create/update endpoints that

   • either form or template field is defined, but not both together.

   For ProjectTemplate we should check in create/update endpoints that:

   • either scope or form field must be defined, but not both together
   • scope or priceConfig cannot be defined together (they can be both null though)
   • either phases or phaseConfig field must be defined, but not both together.

[mfikria]

@maxceem resolved in #297

[maxceem]

@vikasrohit just in case, this is already done and merged to DEV.