Introduce input fields to display current dates assigned to the announcement

We'll assign datepicker to these fields so that they can be edited after the fact.
Slight modification to the language describing archives
Abstract the saving of announcement date meta for use in the admin as well
Add datepicker functionality to the date inputs in the admin

Introduces a small JS file to trigger datepicker
Update the announcement with any new dates chosen in the datepicker

Does not yet delete the old dates.
Clear any announcement date meta associated with a post
Merge pull request #3 from washingtonstateuniversity/edit-dates

Allow for announcement dates to be edited after announcement submission