Changed date format of footer in the Plans pdf export to a more human readable date, ...

e.g., "Last modified 26 April 2019".

Changes:
  - added :readable date format to language locales
  - used this date for the footer in the Plans pdf export.
    Note, changed " formats" to "format" and use :readable in footer date
     From  "date: l(@plan.updated_at.to_date, formats: :short)" -->
     to    "date: l(@plan.updated_at.to_date, format: :readable)"
  - to pass Rspec test spec/features/plans/exports_spec.rb we needed to add
    a locale file en.yml  which is a copy of en-GB.yml to avoid error
    "1) PlansExports User downloads their plan as PDF
     Failure/Error: date: l(@plan.updated_at.to_date, format: :readable)

     I18n::MissingTranslationData:
       translation missing: en.date.formats.readable"

Fix for Issue #2094.

Update CSV downloads with consistent date format ...

Fixes #1936

Added I18n formats date.formats.csv to allow for regional customisation

Sync all translations & locales