CRUD UI for AbsencePeriod

Implementation
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1. civicrm/absence/period
 - show the listing of all the absence peroids in the database with standard actions for 'Edit','Delete'
 - 'Add Absence Period' Button

2. On clicking 'Add Absence Period' Button, Form elements are as follows:
 - Add a textbox for Title
 - Add a date field for Start Date
 - Add a date field for End Date

3. PostProcess :
 On save, the absence peroids are saved in 'civicrm_absence_period' table

Additional Changes:
1) Form Rule on title if chosen title already exists and End Date < Start Date
2) Sort by Start Date on listing