Add a script that check translation lacked or disused

Wrote a script that help translator.

Example:
in OnionShare directory
```sh
$ check_lacked_trans.py
de disused choose_file
de disused gui_starting_server
de lacked gui_canceled
de lacked gui_starting_server1
de lacked gui_starting_server2
de lacked gui_starting_server3
en disused choose_file
es disused choose_file
es disused gui_starting_server
...
```

Format:
[locale] [disused or lacked] [translation_key]

Flow of processing:
1. search `{{strings.translation_key}}` and `strings._('translation_key')`
   from .py or .html files.
2. load translation key from locale/*.json.
3. compare these.