NEAR review

[amgando]

1. refactor string concatenation in the generate function
2. refactor calls to Context.sender to save on gas
3. refactor logging messages into a single logMessage function
4. refactor to add guard clause before calling designs.getSome()
5. refactor Design class by adding a static generate function
6. refactor two Persistent* classes into one
7. surfaced pagination params to viewDesigns contract method