Revise migrations for OLH data

[joemull]

With this pull request, I've revised the migrations so they work well with OLH data.

I've also written a checklist that we can follow when fixing up the data after migrations.

1. Billing agents. Designate one billing agent (OLH) as default. Enter
   countries for the other billing agents: USA for Lyrasis and UK for JISC.

2. Supporter sizes. Set multiplier amounts for the supporter sizes: Large: 1.3,
   Medium: 1.0, Small: 0.7.

3. Supporter levels. Add more levels (Platinum, Gold, Silver, and
   Bronze), and order them. Set standard as default. Leave generic “Higher” for
   now.

4. Add symbols for currencies.

5. Create base bands with JISC as billing agent.

   Platinum
   15000 GBP
   Large university (10,000+ students)
   United Kingdom

   Gold
   10000 GBP
   Large university (10,000+ students)
   United Kingdom

   Silver
   7500 GBP
   Large university (10,000+ students)
   United Kingdom

   Bronze
   5000 GBP
   Large university (10,000+ students)
   United Kingdom

   Standard
   1500 GBP
   Medium university (5,000-9,999 students)
   United Kingdom

6. Match up each higher supporter to its appropriate gold, silver,
   platinum etc. band, or create a fixed-fee band for ones that are far off.
   Ask Paula to check over the results of this process.
   Finally, remove old generic “Higher” level.

7. Run calculate_all_fees command once per year, or as part of process of
   asking universities to become higher supporters. Actual current fee data
   lives in spreadsheets.

8. Run update_demo_band_data so the tables on the Library Publishing
   Fees page are populated.

Closes #47.

[ajrbyers]

👍🏻