Remove the index on Channels.DisplayName. (#8530)

As outlined in [this
discussion](https://pre-release.mattermost.com/core/pl/uw5bwmkb6irkbkn6pk9rkzpytr),
this index causes issues with MySQL's query planner, leading to full
table scans in a case where it would have made more sense to leverage a
filesort.

model/version.go

@@ -9,11 +9,13 @@ import (
 	"strings"
 )
 
-// This is a list of all the current viersions including any patches.
-// It should be maitained in chronological order with most current
+// This is a list of all the current versions including any patches.
+// It should be maintained in chronological order with most current
 // release at the front of the list.
 var versions = []string{
+	"4.8.1",
 	"4.8.0",
+	"4.7.2",
 	"4.7.1",
 	"4.7.0",
 	"4.6.0",