uploads: Only initialize S3 connection once in __init__.
uploads: Only initialize S3 connection once in __init__.

This should be a mild performance optimization for the S3
authentication backend, since we aren't initializing unnecessary
duplicate connections.

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