Add command to show yearly activity map

[TimJentzsch]

Add a command to show an activity map, but for the whole year and not aggregated over the week like /heatmap.

It should look similar to the GitHub activity heatmap:

To obtain the data we can simply use the submission/rate endpoint in Blossom with a daily time frame.

[TimJentzsch]

For the command options, you should be available to provide the user, similar to other commands. It should default to the user executing the command. Providing all to show the activity of everyone should also be possible, similar to other commands.

This will be helpful to see when volunteers are more or less active over the course of a year.