improve de support

[noviluni]

Includes additions from the next PRs:

• Austrian German + dative forms added #348
• Added 'day after tomorrow' and support in German language #650
• Update de.yaml month abbreviations #408
• Added "MRZ" abbreviation for German - month March #400

Most of them modify the json file instead of the yaml file, so we could close them after releasing this.

Also fixes: #649, #395

Fixes #395, fixes #649, closes #348, closes #400, closes #408, closes #650.

[codecov]

Codecov Report

Merging #673 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #673   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files         302      302           
  Lines        2558     2558           
=======================================
  Hits         2436     2436           
  Misses        122      122           

Impacted Files	Coverage Δ
dateparser/data/date_translation_data/de.py	100.00% <ø> (ø)
dateparser/data/date_translation_data/en.py	100.00% <ø> (ø)

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