Adapt label of myActivities filter button depending on state

[simfeld]

          Suggestion: conditional label "Nur meine Aktivitäten/Only my activities" (if no other filters set) or "Alle meine Aktivitäten/All my activities" (if other filters are applied) to make it clear that all other filters will be reset.

Originally posted by @simfeld in https://github.com/ecamp/ecamp3/pull/4573#discussion_r1564298295