Fix: Refactor yield distribution to single transfer instead of double

[RonTuretzky]

Imo, an improved design would be returning the amount to distribute from the _distributeBaseYield and _distributedVotedYield functions and doing a single transfer with their sum. Reduces gas costs and improves readability by allowing these two functions to be changed to "view" to ensure no secret state changes. @bagelface

• [ ] Changing both functions to view functions
• [ ] Creating a parent transfer function for each project instead of two transfers
• [ ] Reducing any other redundant iterations
• [ ] Introducing tests to each function

[bagelface]

Adding tests deferred to ticket https://github.com/BreadchainCoop/breadchain/issues/21