Supporter record management

[mauromsl]

I fat fingered the merge button on #65 without a second reviewer

This closes #63 and closes #54 as well as completing some necessary yak shaving:

• Simplifies the types of bands into a choice field called "category". Bands can now be base, calculated, or special.
• Updates the logic and utils methods to use this field
• Updates the tests in line with logic and utils, and while we're at it, to be more readable and maintainable
• Starts work on As a press manager I want to manage bands from year to year #64 in the models and utils (e.g. prospective_band and get_total_revenue)

TIP! You can quickly get up and running with some sample data by running the management
command install_example_supporter_data.

[ajrbyers]

I've added a comment about the ROR API rate limit for the management command. Its possible if the command runs fast enough that it might hit the limit.