/top-helpers should allow month selection

[Zabuzard]

Overview

Sometimes, we want to use /top-helpers not only to actually compute and select the top helpers for the last month, but we also want to just have a glimpse at the board for the current month. Sometimes, we maybe also want to look in the past.

Originally, we wanted to wait for a Discord-built-in date/time-selection, but it seems that this will take a bit longer still.

Proposed solution

A simple solution to this could be to add an optional month-parameter (drop down with strings for all months). If a month is selected, we compute the ranking for that month instead.

Year-wise, if the month is before-equals the current month, we pick the current year. If the month is after the current month, we pick the previous year. So /top-helpers month:November, being used in March 2022, would result in November 2021.