Store LEKs (Social Encryption Keys)

[Super-Baleine]

ref. muonium/webclient#11

1. get LEK from the webclient
2. store it into files -> LEK as a map such as: {'user_id': 'LEK_enc'}

With several LEKs, we obtain such as json structure:

{'user_id0': 'LEK_enc0', 'user_id1': 'LEK_enc1', ..., 'user_idX': 'LEK_encX'}

DON'T SEND ALL THE MAP TO THE WEBCLIENT! PARSE IT AND GIVE ONLY THE LEK WHICH GOT ENCRYPTED THANKS TO HIS PUBLIC KEY!