Various improvements and fixes (#522)

Various improvements and fixes (#522)

* set default startup volume for airplay

* fix delete provider order

* fix zeroconf discovery

* fix some typos

* reduce save delay

* extend prebuffer amount

* prevent duplicate tasks: prevent the same media item being added at the same time

* Fix issues with json serializer and encrypted passwords

* send changed keys to player changed callback

* rework json serializer

* add ca-certificates to dockerfile

* update documentation links

* bump frontend to 20230313.0