Increase Channel Purpose length to 250, and add channel field length handling code to the slack importer (#4458)

* Increase Channel Purpose length to 250.

This commit increases the maxmimum length of the channel purpose field
to 250, including the database migration necessary.

It also adds a method to the Slack importer to check the lengths of
channel properties before importing, and truncate them if necessary.

Fixes #4168

* Fix database migration.

store/sql_channel_store.go

@@ -38,7 +38,7 @@ func NewSqlChannelStore(sqlStore *SqlStore) ChannelStore {
 		table.ColMap("Name").SetMaxSize(64)
 		table.SetUniqueTogether("Name", "TeamId")
 		table.ColMap("Header").SetMaxSize(1024)
-		table.ColMap("Purpose").SetMaxSize(128)
+		table.ColMap("Purpose").SetMaxSize(250)
 		table.ColMap("CreatorId").SetMaxSize(26)
 
 		tablem := db.AddTableWithName(model.ChannelMember{}, "ChannelMembers").SetKeys(false, "ChannelId", "UserId")