decorator: Add decorator for checking whether user is server admin.
decorator: Add decorator for checking whether user is server admin.

This is just variabnt of `require_server_admin` for JSON/api views.

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