Remove Cozy support

Version 0.13 will remove Cozy support. Here are the items we can remove that come to mind:

• support for cozydb
• emails sent by the cozy helper
• support for file attachments onto transactions
• notifications (unless we implement an external system)
• standalone vs cozy mode on the server
• https://framagit.org/bnjbvr/kresus/blob/0a0641c9ef43673bef802600b82beaf0ff244ad3/server/models/alert.js#L81
• permissions and cozy descriptors in package.json

Feel free to update this list and add other items!