slack: Rename slack_custom_field_name_to_zulip_custom_field_id.
slack: Rename slack_custom_field_name_to_zulip_custom_field_id.

Rename custom_field_map to
slack_custom_field_name_to_zulip_custom_field_id.

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