Internationalization (German) of the mobile app. (#246)

* Add i18n framework to mobile app and write simple translation generator

* Replace all texts in login_form with i18n keys

* Localization of sharing section

* Localization of asset viewer section

* Use JSON as base translation format

* Add check for missing/unused translation keys

* Add localizely

* Remove i18n directory in favour of localizely

* Backup Translation

* More translations

* Translate home page

* Translation of search page

* Translate new server version announcement

* Reformat code

* Fix typo in german translation

* Update englisch translations

* Change translation keys to match dart filenames

* Add /api to translated endpoint_urls

* Update localizely.yml

* Add languages to ios plist

* Remove unused keys

* Added script to check outdated key in other translations

* Add download key to localizely.yml

Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/ios/Runner/Info.plist

@@ -82,5 +82,11 @@
     <array>
       <string>https</string>
     </array>
+
+    <key>CFBundleLocalizations</key>
+    <array>
+      <string>en</string>
+      <string>de</string>
+    </array>
   </dict>
 </plist>