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* Added script to check outdated key in other translations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/scripts/check_i18n_keys.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+import json
+import subprocess
+
+def main():
+    with open('assets/i18n/en-US.json', 'r') as f:
+        data = json.load(f)
+
+        for k in data.keys():
+            print(k)
+            sp = subprocess.run(['sh', '-c', f'grep -r --include="*.dart" "{k}"'])
+
+            if sp.returncode != 0:
+                print("Not found in source code!")
+                return 1
+
+if __name__ == '__main__':
+    main()