Case-sensitive handling of subject reply prefix

## What Happened
Mail has case-sensitive detection of the "Re: " prefix in a message subject. Some mail clients, Microsoft Outlook's desktop client definitely being one of them append a capital "RE: ". If you reply to one of these messages with Mail, you can end up with an undesirable "Re: RE:" chain on the front of the subject. This can grow to silly lengths if both clients are case-sensitive when they try and detect the prefix.

## Expected Behavior
The "Re: " prefix is detected regardless of case.

## Additional Notes
While Mail does not currently support prepending a "Fwd:" prefix, some mail clients handle this different again. For example, Microsoft Outlook prepends "FW:"

## Platform Information
elementary OS 6.0 Beta