Add Admin model

bin/rails g model admin full_name:string username:string:uniq:index password_digest:string deletable:boolean is_active:boolean
Migrate the DB
ready Applicant model
removed Present validators from applicants.rb
group validators in API::V1::Validators
fix grape validator missing error
add grape entity for Admin model
add main REST actions for admins endpoint
almost finish admins endpoint main actions
add confirmation grape custom validator