feat(localization): add German localization

[mkoehne]

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Description of the pull request

Added German localization.

[DaemonLoki]

Added almost all of my corrections as suggestions so that you can quickly use them. :)

Wonderful day!
Oh, and what do you call a sorceress in the desert? ....a sandwitch

[codecov]

Codecov Report

Merging #1114 (fcdd62f) into develop (9204a62) will increase coverage by 0.17%.
The diff coverage is 93.98%.

@@             Coverage Diff             @@
##           develop    #1114      +/-   ##
===========================================
+ Coverage    67.76%   67.94%   +0.17%     
===========================================
  Files          228      229       +1     
  Lines        23462    23588     +126     
===========================================
+ Hits         15899    16026     +127     
+ Misses        7563     7562       -1     

Impacted Files	Coverage Δ
...izations/lib/src/stream_chat_localizations_de.dart	93.93% <93.93%> (ø)
...calizations/lib/src/stream_chat_localizations.dart	100.00% <100.00%> (ø)
...es/stream_chat_flutter/lib/src/channel_header.dart	100.00% <0.00%> (ø)
...src/v4/message_input/stream_attachment_picker.dart	33.47% <0.00%> (+0.28%)	⬆️
...es/stream_chat_flutter/lib/src/message_widget.dart	31.21% <0.00%> (+0.56%)	⬆️

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