Change default download location

`/url save $someurl` will now download to
`~/.local/share/profanity/downloads/from_jid/date/filename` instead of
`~/.local/share/profanity/downloads`.

Like this the downloaded files should be better ordered.

This will only happen for MUC and 1:1 chat windows.
Private windows might have only a nick or jid. Lets not distinguish
between those two ways. And since the nick option is unreliable to be
the same person lets just put them in the general downloads folder.
As is done with downloads from all other windows as well.

I also had the idea to make this configurable but this suits my needs
and time limits right now.

src/command/cmd_defs.c

@@ -2572,7 +2572,7 @@ static const struct cmd_t command_defs[] = {
               "Open or save URLs. This works with OMEMO encrypted files as well.")
       CMD_ARGS(
               { "open", "Open URL with predefined executable." },
-              { "save", "Save URL to optional path, default path is current directory." })
+              { "save", "Save URL to optional path. The location is displayed after successful download." })
       CMD_EXAMPLES(
               "/url open https://profanity-im.github.io",
               "/url save https://profanity-im.github.io/guide/latest/userguide.html /home/user/Download/")