Change habtm to hmt

[#69540418]
Refactor - group by resource

[#69540418]
Category should have many incomes

[#69617754]
Use rails to check for presence

[#69618250]
Merge pull request #83 from bignerdranch/refactor_habtm

Refactor to hmt - [#69540418]
Merge pull request #87 from bignerdranch/refactor_category_income_assoc

Category should have many incomes - [#69617754]
Merge pull request #88 from bignerdranch/refactor_use_rails_validations

Use rails to check for presence - [#69618250]

:tada:
Add db level uniqueness constraint

[#69618172]
Add db validation for presence

[#69618172]