A site manager can configure the date format to use for acknowledgement messages

Problem/Motivation

A date format should not be hard-coded. Drupal has a fine Date and time formats capability.

Proposed resolution

We should:

1. Add a configuration setting for the date format.
2. Ship a configuration (or require a module dedicated to shipping this configuration, nothing but an info file and a few datetime format files in config/install) with the same format we currently use, Month day, YEAR, which is the same as Drutopia ships with – https://git.drupalcode.org/project/drutopia_core/blob/2.0.x/config/insta... – and if we do go the contrib module route, Drutopia would switch to shipping with it too instead of defining its own custom format there.
3. Set our default for the setting to this format.

Remaining tasks

User interface changes

API changes

Data model changes