add flow ordering field to UIGroup model

[hburgund]

In order to make the process of recording for users as smooth and intuitive as possible, it will be very nice to allow for some UIGroups to be presented to the user prior to making their recording (as is done now with all UIGroups) and some to be presented after the recording is completed. This post-record tagging could be done immediately after the recording is made or as a part of the upload process along with adding additional media etc.

In order to allow for this flexibility, UIGroups need to have a field to say whether they should be presented pre- or post-record.

This new field uigroup.flow_position (better name?) should have two options: pre-record and post-record and is required with a default of pre-record.