It was there since the C core, labeled with "/* last_seen is for
future use */" but never actually used. The comment was lost during
the translation from C to Rust.
`cutil.from_dc_charpointer()` is guaranteed to return `str`, while
`cutil.from_optional_dc_charpointer()` may return `None` if C function
returns `NULL`.
Fix#1791 and show all group messages if the user already is in the group, even if the sender is blocked
Also fix a comment
Co-authored-by: link2xt <ilabdsf@gmail.com>