End of pay period balance budgets

[jantman]

Add a modal dialog for balancing the previous pay periods' budgets. This should only happen for the immediate last period, i.e. the period before the current one.

The idea is that this will go through each periodic budget in that pay period with a remaining balance not zero, and transfer funds between periodic budgets (using the last date of the pay period as the transfer date) to attempt to get them all to a zero balance, and to get the overall "remaining" amount for the period to zero. If the overall period is negative, after zeroing all positive periodic budgets, funds will be transferred from a user-selected standing budget to zero all negative periodic budgets. If any periodic budgets are positive after balancing, the funds will be transferred to the user-selected standing budget.

The end goal for this is for all pay periods to close (i.e. the day after the pay period is over) with a $0 remaining value, i.e. all income has been allocated to a budget and all allocations were funded by income or a standing budget. The hope is that at the end of each pay period, the balance of standing budgets will match the balance of all budget-funding accounts.

1. Click a button on a past period, and it pops up a modal prompting the user to select a standing budget to use for excess or underage funds. User selects standing budget and clicks ok; modal submits to backend.
2. Modal refereshes with a table of the current "remaining" value of each periodic budget and the future "remaining" value of each periodic budget, plus the current and future values of the selected source standing budget. It also includes textual description (li elements) of each transfer to make, and then an "OK" button at the bottom that POSTs that back and makes the transfers.

Outstanding items:

• Add to screenshots
• Only allow balancing on immediately previous payperiod

[jantman]

Important: I also want to get the whole/overall Pay Period Remaining value to zero. Use http://192.168.0.24:5000/payperiod/2017-09-29 (before transfers) as an example.