Add second slider to AgeSlider component for choosing retirement month

[dmeegan]

This pull request:

1. Adds a second slider for choosing the desired retirement month.
2. Revisises calculations to change on setting both desired year(age) and month(age) of retirement.
3. Disables the month slider and sets the desired month in the calculation to zero when retirement age is set to 70.

Is progress on issue #115

[thadk]

Can you please add ticks to the slider so Zero says January, 6 says June and 11 says December? I did this for FRA in the original slider, so you can use that as an example.

[dmeegan]

As of my latest commit at 11 on 11-20, the slider still needs to be worked on. Currently, it automatically sets the months to 0 or 1 based on 62 or 70 years old, however, the month stays at that age when switching to other years.