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.

model/channel_test.go

@@ -78,7 +78,7 @@ func TestChannelIsValid(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	o.Purpose = strings.Repeat("01234567890", 20)
+	o.Purpose = strings.Repeat("01234567890", 30)
 	if err := o.IsValid(); err == nil {
 		t.Fatal("should be invalid")
 	}
@@ -87,6 +87,11 @@ func TestChannelIsValid(t *testing.T) {
 	if err := o.IsValid(); err != nil {
 		t.Fatal(err)
 	}
+
+	o.Purpose = strings.Repeat("0123456789", 25)
+	if err := o.IsValid(); err != nil {
+		t.Fatal(err)
+	}
 }
 
 func TestChannelPreSave(t *testing.T) {