Go back to sg_ as username tmplate

[rosbeef]

Problem

• User name template change from sg_IDXYZ to signal_XYZ wich make a mess in chats

Solution

• Go back to sg_IDXYZ the early as possible to not make que mess keep occuring on new messages

PR Status

• Code finished and ready to be reviewed/tested
• The fix/enhancement were manually tested (if applicable)

[Gredin67]

I've just had days with no working signal bridge because of this. I changed it to sg_ after upgrade, but forgot to change the app registration file too..

With noone opposing in #135 I would merge