Couple of changes to tidy up UI.
Couple of changes to tidy up UI.

- Remove blank "-------" values from select boxes (except Tracker
  select box in add_tracker_url and edit_tracker_url).
- Add __str__ method to BugzillaTracker model so select box in
  add_tracer_url and edit_tracker_url displays human-readable names.

ehashman/oh-mainline,campbe13/openhatch,heeraj123/oh-mainline,nirmeshk/oh-mainline,ehashman/oh-mainline,onceuponatimeforever/oh-mainline,sudheesh001/oh-mainline,Changaco/oh-mainline,heeraj123/oh-mainline,vipul-sharma20/oh-mainline,vipul-sharma20/oh-mainline,jledbetter/openhatch,eeshangarg/oh-mainline,jledbetter/openhatch,onceuponatimeforever/oh-mainline,willingc/oh-mainline,nirmeshk/oh-mainline,campbe13/openhatch,openhatch/oh-mainline,jledbetter/openhatch,eeshangarg/oh-mainline,mzdaniel/oh-mainline,moijes12/oh-mainline,onceuponatimeforever/oh-mainline,willingc/oh-mainline,mzdaniel/oh-mainline,nirmeshk/oh-mainline,onceuponatimeforever/oh-mainline,SnappleCap/oh-mainline,ojengwa/oh-mainline,heeraj123/oh-mainline,ojengwa/oh-mainline,mzdaniel/oh-mainline,campbe13/openhatch,SnappleCap/oh-mainline,vipul-sharma20/oh-mainline,Changaco/oh-mainline,vipul-sharma20/oh-mainline,openhatch/oh-mainline,waseem18/oh-mainline,ehashman/oh-mainline,eeshangarg/oh-mainline,campbe13/openhatch,jledbetter/openhatch,ojengwa/oh-mainline,onceuponatimeforever/oh-mainline,sudheesh001/oh-mainline,nirmeshk/oh-mainline,moijes12/oh-mainline,SnappleCap/oh-mainline,nirmeshk/oh-mainline,SnappleCap/oh-mainline,ojengwa/oh-mainline,vipul-sharma20/oh-mainline,Changaco/oh-mainline,mzdaniel/oh-mainline,waseem18/oh-mainline,willingc/oh-mainline,ehashman/oh-mainline,Changaco/oh-mainline,moijes12/oh-mainline,Changaco/oh-mainline,sudheesh001/oh-mainline,waseem18/oh-mainline,campbe13/openhatch,willingc/oh-mainline,SnappleCap/oh-mainline,jledbetter/openhatch,waseem18/oh-mainline,openhatch/oh-mainline,moijes12/oh-mainline,waseem18/oh-mainline,mzdaniel/oh-mainline,ojengwa/oh-mainline,sudheesh001/oh-mainline,sudheesh001/oh-mainline,moijes12/oh-mainline,heeraj123/oh-mainline,willingc/oh-mainline,heeraj123/oh-mainline,mzdaniel/oh-mainline,eeshangarg/oh-mainline,openhatch/oh-mainline,openhatch/oh-mainline,eeshangarg/oh-mainline,mzdaniel/oh-mainline,ehashman/oh-mainline