Persist usage start and end dates in the session

This patch adds the usage start and end dates to the user's session
so they are saved while navigating between pages.

Closes-Bug: #1373651
Change-Id: I3046b5533eb96f85fb2f90e63b3ff84360ad14c4
Use identity operator in horizon.modals.js

jshint logs usage of the equality operator as an error.

Change-Id: Ia5478981f6ce87e77c65bcc569ae542eeeff099f
Closes-Bug: #1375401
Imported Translations from Transifex

Change-Id: Ic411f3e892bfbfef4fba0723b63e01cc7b8939fd
Merge "Persist usage start and end dates in the session"
Merge "Use identity operator in horizon.modals.js"
Merge "Imported Translations from Transifex"