Allow purging of notifications and programs

[higs4281]

We need to be able to purge notification and program objects via Jenkins in order to prepare for live testing by a school. This adds manage.py commands for doing that, plus a model function for ensuring that program_length values that are delivered as json are always even.

Additions

• purge command and purge_objects script to support it
• related tests
• model function to deliver even values for program_lengths

  Testing

• Try wiping programs and notifications from your local db.

./manage.py purge notifications
./manage.py purge programs

Review

• @amymok and any purge-happy folk

  Checklist

• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[coveralls]

Coverage increased (+0.02%) to 99.384% when pulling ad2e64543952c67bb4b17a063d9b476c69f4c6ce on purges into e48bd9e37193375b59691701461f85ae79f85717 on master.

[coveralls]

Coverage increased (+0.02%) to 99.384% when pulling 86ede91ea3be4ca20f46433d0c7dd2b5ab6d629a on purges into e48bd9e37193375b59691701461f85ae79f85717 on master.

[amymok]

Other than the comments above, everything looks good~

[coveralls]

Coverage increased (+0.02%) to 99.384% when pulling 00349646016ee3bfc08410f46dbc08c7f5bb471a on purges into e48bd9e37193375b59691701461f85ae79f85717 on master.