MM-29573: Reuse s3 client amongst all requests (#15948)

* MM-29573: Reuse s3 client amongst all requests

An http.Client is meant as a long-term object to be reused between
consecutive requests. This reduces burden on HTTP servers by reusing TCP connections
and also makes things faster by avoiding the TCP handshake for every single request.

https://mattermost.atlassian.net/browse/MM-29573

* Trigger CI

* Fix i18n

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

go.tools.sum

@@ -70,6 +70,10 @@ github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200915114419-f4421b
 github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200915114419-f4421bc07461/go.mod h1:3gKozJI8n2Y/vW37GfnFWAdehGXe5yZlt+HykK6Y3DM=
 github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200926180007-fd1b679200e5 h1:1fVtMi+1XPAtxffRZiMSiy7Zm4Od3fyHWnC2BydfYHY=
 github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200926180007-fd1b679200e5/go.mod h1:3gKozJI8n2Y/vW37GfnFWAdehGXe5yZlt+HykK6Y3DM=
+github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20201001222518-66764584f346 h1:ByexaM3lb5ZdHGtcB1QwRkDXDxt/2MUTvI8+LjwhDXg=
+github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20201001222518-66764584f346/go.mod h1:3gKozJI8n2Y/vW37GfnFWAdehGXe5yZlt+HykK6Y3DM=
+github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20201013204121-e463fcd00ba8 h1:6wiKDdS/MMY0Jht12lRZJq1zv5eHEd4fK60SwkUVGjA=
+github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20201013204121-e463fcd00ba8/go.mod h1:3gKozJI8n2Y/vW37GfnFWAdehGXe5yZlt+HykK6Y3DM=
 github.com/matttproud/golang_protobuf_extensions v1.0.1/go.mod h1:D8He9yQNgCq6Z5Ld7szi9bcBfOoFv/3dc6xSMkL2PC0=
 github.com/mitchellh/go-homedir v1.1.0/go.mod h1:SfyaCUpYCn1Vlf4IUYiD9fPX4A5wJrkLzIz1N1q0pr0=
 github.com/mitchellh/mapstructure v1.1.2/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=