Validate Project Settings values

[maxceem]

We implemented a ProjectSettings model with CRUD endpoints.

Currently, we support the next values types:

  INT: 'int',
  DOUBLE: 'double',
  STRING: 'string',
  PERCENTAGE: 'percentage',

We should validate values when we create/update records, based on their types:

  INT: check that it's integer
  DOUBLE: check that it's double
  PERCENTAGE: check that it's a value from `0` to `100`

Also, we should enable existent unit tests for validating percentage value type and create unit tests for other types of values.