get_recipient_info: Simplify user_ids_muting_topic handling.
get_recipient_info: Simplify user_ids_muting_topic handling.

Rather than subtracting sets in multiple places, it's simpler/cleaner
to just check which users are in the set when processing them.

This refactoring be helpful when we extend the get_recipient_info
logic to handle wildcard mentions as well.

andersk/zulip,andersk/zulip,timabbott/zulip,brainwane/zulip,shubhamdhama/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,hackerkid/zulip,showell/zulip,rht/zulip,brainwane/zulip,rht/zulip,eeshangarg/zulip,timabbott/zulip,showell/zulip,kou/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,showell/zulip,brainwane/zulip,brainwane/zulip,punchagan/zulip,rht/zulip,timabbott/zulip,showell/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,hackerkid/zulip,shubhamdhama/zulip,shubhamdhama/zulip,punchagan/zulip,shubhamdhama/zulip,synicalsyntax/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip