Store/cache chat messages in RS

[raucao]

We can use and adapt the existing chat-messages module for storing the logs locally, and (later) syncing them to/from one's RS account: https://github.com/67P/remotestorage-module-chat-messages

[raucao]

I moved the RS chat-messages module to our Gitea, and updated the data model there:

https://gitea.kosmos.org/kosmos/rs-module-chat-messages/pulls/4