vnstat expose MonthRotate in UI

[gpfountz]

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Is your feature request related to a problem? Please describe.
My ISP has a monthly data cap and the cap resets on the 14th of the month.

Describe the solution you'd like
I would like the UI to allow vnstat's monthly totals to reset on a user selectable day of the month instead of the 1st.

Describe alternatives you've considered
I tried modifying /usr/local/etc/vnstat.conf changing MonthRotate to 14. But, this reset back to 1 after applying a normal update.

[gpfountz]

I have completed the changes for this feature request.

If you approve this feature and are interested in accepting changes from a novice contributor, I will create a pull request.

[mimugmail]

Sure, just open a PR for review