MM-10417 Add local image proxy and enable by default (#9967)

* MM-10417 Add local image proxy and enable by default

* Remove unused function

* Add dependencies for willnorris/imageproxy

* Fixed compilation errors

* Lock to the master version of willnorris/imageproxy

* Fix atmos/camo proxy when no SiteURL is specified

* Re-add default values for deprecated settings

* Fix unit tests added by merge

* Pass imageproxy to App struct

* Remove unneeded locking when creating the image proxy

* Remove empty test file

app/app.go

@@ -14,6 +14,7 @@ import (
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
 	"github.com/mattermost/mattermost-server/services/httpservice"
+	"github.com/mattermost/mattermost-server/services/imageproxy"
 	"github.com/mattermost/mattermost-server/services/timezones"
 	"github.com/mattermost/mattermost-server/utils"
 	goi18n "github.com/nicksnyder/go-i18n/i18n"
@@ -43,6 +44,7 @@ type App struct {
 	Saml             einterfaces.SamlInterface
 
 	HTTPService httpservice.HTTPService
+	ImageProxy  *imageproxy.ImageProxy
 	Timezones   *timezones.Timezones
 }