show device message only once as unread

[bjoern]

on switching or adding profiles,
the device messages (welcome and update messages) are added - they are maked as unread even though maybe read already several times, being unnecessary noise, not only for us devs.

we should add them as "read" instead, if they're read in other profiles already.

(this started on android with the idea to not add the messages, however, it turned out that it is easier and less confusing to make the messages as read)

implementation-wise, both, android and ios:

• check if a global user-setting is equal to the deviceMessageLabel just added
• if so, the chat is marked as read completely
• after that, unconditionally, deviceMessageLabel is set

reference implementations: https://github.com/deltachat/deltachat-ios/pull/2204/files, https://github.com/deltachat/deltachat-android/pull/3069/files ("deviecMsgId" should be named "deviceMsgLabel" there cmp)

this issue has low priority