Add presence validations for required fields (#545)

* Adds basic validations for required fields

Also deletes a few extraneous .keep files

Does not add the family_id required field for user, since that breaks the basic test setup

* Restore keep files to this branch

* Remove Credit model and validate models behind ids

* Restore concerns .keep

app/models/valuation.rb

@@ -1,6 +1,8 @@
 class Valuation < ApplicationRecord
   belongs_to :account
 
+  validates :account, :date, :value, presence: true
+
   after_commit :sync_account
 
   scope :in_period, ->(period) { period.date_range.nil? ? all : where(date: period.date_range) }