Give feedback on subscribed holiday calendar

[jancborchardt]

Is your feature request related to a problem? Please describe.

Currently when you subscribe to a holiday calendar, the "Subscribe" button only disables, making it unsure if something happened (unless you look into the sidebar and see the calendar).

Describe the solution you'd like

It would be good to change the text to "Subscribed" as feedback. As discussed @ChristophWurst

Describe alternatives you've considered

No response

Additional context

#5167