minor and major reworks: rename swb to SRU, add a test for pdf parsing

major: rework mail to send mail as plaintext instead of html, preventing the bleed-in of html text

src/ui/widgets/signature_update.py

@@ -8,7 +8,7 @@ from PySide6.QtMultimedia import QAudioOutput, QMediaPlayer
 from src import LOG_DIR
 from src.backend.catalogue import Catalogue
 from src.backend.database import Database
-from src.logic.swb import SWB
+from src.logic.SRU import SWB
 
 from .widget_sources.admin_update_signatures_ui import Ui_Dialog