misc fixes - Travel Request : mileage history added, fixed: approve button should not appear in (i)applicant show pg & (ii)when NOT YET submitted [replace_by & hod_id not exist]
travel request - model - required data setting for mileage history
Travel Request - hods & replacement revised
Merge pull request #316 from Nurhashimah/master

Travel Request - Mileage & Mileage Replace (revised) + Mileage history added & PDF format revised