"Sessions" page fixes

[tieneupin]

• When viewing a single session:
  • "Resume" doesn't appear in the drop-down menu on a paused Rsyncer
  • "Remove" and "Finalise" could be changed to be more straightforward
    • Remove --> Delete Rsyncer (Delete from database and remove from 'Sessions' page)
    • Finalise --> Delete Rsyncer and Raw Data (additionally runs rsync with '--remove-source-files' flag)
  • "Visit Complete" doesn't delete session-related entries from database
• On the "Sessions" page:
  • "Delete session" button currently doesn't work (expected to remove session from list)