Refactor ProductTemplates to use Form instead of template property

[maxceem]

We already refactored ProjectTemplate model to keep form templates inside Form model.
We want to do the same for ProductTemplate model.

1. Make property ProductTemplate.template optional.
2. Add property ProductTemplate.form which will refer a particular form version same like ProjectTemplate: { key: String, version: Number }
3. Create a DB migration script for changes in ProductTemplate model inside /migrations folder
4. Update /projects/metadata?includeAllRefered when we define includeAllRefered param it should also return forms which are referred in ProductTemplates.
5. Create a migration script similar to https://github.com/topcoder-platform/tc-project-service/blob/dev/src/routes/projectTemplates/upgrade.js which we can run by calling POST /v4/projects/metadata/productTemplate/:templateId(\\d+)/upgrade.
   The migration script would look like this:
   • if Form is not specified: create a new Form and copy JSON from ProductTemplate.template to Form.config
   • set ProductTemplate.form: { key: Form.key, version: Form.version }
   • set ProductTemplate.template to null
6. Update all unit tests, swagger and postman files according to new changes.

[maxceem]

@vikasrohit @RishiRajSahu as was mentioned by vikas here #283 we should do refactoring for productTemplates same like we did for projectTemplates.

Could you please confirm that this is desired change and I'll run F2F immediately.

[RishiRajSahu]

@maxceem yeah we can launch it, but does it impact being run in parallel the execution of other backend/frontend challenges ?

[maxceem]

@RishiRajSahu this change so far is backward compatible, so it shouldn't affect other tasks in parallel.

[vikasrohit]

@maxceem I think we are done with this, right?

[maxceem]

@vikasrohit right, it's done.