Add visibilty options (hide/opacity) for month overlapping days

Problem/Motivation

There is no option to hide or style days, which are overlapping a month. I need this for a vacation calendar and it is a little bit confussing if these days are displayed.

Proposed resolution

I added these options to the calendar format settings form and added conditions to add css classes to the month overlapping days.

If "yes" is selected a css class with visibility: hidden; is added to these days.

If "no" is selected a new form select element appears with opacity values in 10% steps. A css class will be added with the chosen value. For example 30% addsopacity: 0.3;

I reused parts of the orignally code am not sure if it is the best way to do this.

btw this is my first issue/patch so feel free to point me in the correct direction if something should be different.

Remaining tasks

review needed, better naming of the elements/descriptions

User interface changes

Added two options to the calendar format settings. Radio buttons to hide month overlapping days and a select field to specify the opacity of these days if you didn't want to hide them completely.