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.

api/channel_test.go

@@ -546,7 +546,7 @@ func TestUpdateChannelPurpose(t *testing.T) {
 	}
 
 	data["channel_id"] = channel1.Id
-	data["channel_purpose"] = strings.Repeat("a", 150)
+	data["channel_purpose"] = strings.Repeat("a", 350)
 	if _, err := Client.UpdateChannelPurpose(data); err == nil {
 		t.Fatal("should have errored on bad channel purpose")
 	}