add basic login action to admins endpoint with specs
move some duplicated code in endpoints to the Defaults module
refactor
moved all helpers to Defaults module
add a dedicated Helpers module
add SharedParams module
remove unused code in admins endpoint
add params module for each endpoint
reorganize shared params' modules
move all helper to API::V1::Helpers module

- move all params to API::V1::Params module
update gems
prepare to sperate login specs into its own shared spec
remove login spec from admins_spec
add controllers/login shared_spec
use login shared spec in admins_spec
refactor
add Loginable module for models with username and password attrs
use Loginable in Admin model and use its constants to define roles