Invoice updates intermediate PR

Figure I should make a PR before this feature gets any larger.
- fixes some edge cases for incomplete coverage months and community invoice generation
- changes how community invoices are generated
- adds a skeleton view for user-facing invoices page (view in another PR)
- adds hook for email (implementation in another PR)
- adds migrations for CreditLines being according to product or feature types rather than a specific rate
- fixes tests to reflect above changes