FlagTypes convert t() to $this->t()

I just want to do a small amounts of housekeeping as I go

I could create an issue to do this throughout the module, but I would rather keep the maintenance and review burden to small bite sized chunks

So I am just going to convert the calls to t() to a small localized area.

Plugin/Flag all the FlagTypes need converting.

All the FlagTypes need string translation at the moment, so it seems an appropriate thing to add the the FlagTypeBase class.