MM-38634: Simplify image proxy (#19603)

We copy over the necessary code to simplify
the local image proxy feature.

https://mattermost.atlassian.net/browse/MM-38634

```release-note
NONE
```

go.mod

@@ -32,6 +32,7 @@ require (
 	github.com/go-sql-driver/mysql v1.6.0
 	github.com/golang-migrate/migrate/v4 v4.15.1
 	github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0
+	github.com/gomodule/redigo v2.0.0+incompatible // indirect
 	github.com/google/btree v1.0.1 // indirect
 	github.com/gopherjs/gopherjs v0.0.0-20220104163920-15ed2e8cf2bd // indirect
 	github.com/gorilla/handlers v1.5.1
@@ -123,7 +124,6 @@ require (
 	gopkg.in/mail.v2 v2.3.1
 	gopkg.in/olivere/elastic.v6 v6.2.37
 	gopkg.in/yaml.v2 v2.4.0
-	willnorris.com/go/imageproxy v0.11.2
 )
 
 // Hack to prevent the willf/bitset module from being upgraded to 1.2.0.