invite: Refactor update_subscription_checkboxes.
invite: Refactor update_subscription_checkboxes.

This is a preliminary commit for further changes.
In refactoring, logic for gettings streams is extracted.

brainwane/zulip,showell/zulip,rht/zulip,tommyip/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,timabbott/zulip,timabbott/zulip,synicalsyntax/zulip,dhcrzf/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,rht/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,rishig/zulip,shubhamdhama/zulip,shubhamdhama/zulip,brainwane/zulip,dhcrzf/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,rht/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,timabbott/zulip,synicalsyntax/zulip,tommyip/zulip,rht/zulip,synicalsyntax/zulip,rht/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,timabbott/zulip,rishig/zulip,synicalsyntax/zulip,synicalsyntax/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,jackrzhang/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,dhcrzf/zulip,eeshangarg/zulip,dhcrzf/zulip,punchagan/zulip,zulip/zulip,rishig/zulip,showell/zulip,showell/zulip,rishig/zulip,dhcrzf/zulip,zulip/zulip,hackerkid/zulip,rishig/zulip,jackrzhang/zulip,jackrzhang/zulip,jackrzhang/zulip,punchagan/zulip,synicalsyntax/zulip,rishig/zulip,tommyip/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,tommyip/zulip,jackrzhang/zulip,hackerkid/zulip,zulip/zulip,rishig/zulip,hackerkid/zulip,timabbott/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,eeshangarg/zulip,jackrzhang/zulip,andersk/zulip,kou/zulip,brainwane/zulip,tommyip/zulip,showell/zulip,hackerkid/zulip,synicalsyntax/zulip