Share pictures, files, code snippets, etc. via RS

[raucao]

This one needs multiple separate issues, because sharing a code snippet will need different UI and function calls than sharing an image file for example. Just adding this as a tracking issue, so we have it on the roadmap.

See https://wiki.kosmos.org/Feature:File_sharing for detailed feature description.

Note: in order to add history/search not just for one's own shared files, but other people's shares, the daemon (currently botka) will need to index/store those URLs + meta data to the channel's associated RS account.