Add an upstream issue field (closes issue #233)

This adds an upstream issue field into Issue model. It is viewable in the side panel.

Demo: http://codereview-keydown.appspot.com/1/

Questions:
- better interface (growth of field list overloads interface)
- upload.py support
  - one more option and upload.py help won't fit one screen
  - chromium folks are using BUG=... property autolinking in issue description

[Andi Albrecht, 2010-10-29 19:41:56]

I'm still not sure if we really need such an field. If I understand it correctly
then it's only purpose is to link back to a issue in a bug tracker or similar
system. But I'm not sure if it's enough to mention such references in the
description. Is it always 1:1? OTOH I don't see strong arguments against it,
except the UI issue and if we decide to add it then it should be added as an
option to upload.py too.

[Andi Albrecht, 2010-10-29 19:42:35]

[...and now for the comments :)]

http://codereview.appspot.com/2790041/diff/1/app.yaml
File app.yaml (right):

http://codereview.appspot.com/2790041/diff/1/app.yaml#newcode2
app.yaml:2: version: 31
No need to update the version here.

http://codereview.appspot.com/2790041/diff/1/codereview/models.py
File codereview/models.py (right):

http://codereview.appspot.com/2790041/diff/1/codereview/models.py#newcode83
codereview/models.py:83: upstream = db.LinkProperty()
LinkProperty is very restrictive, at least the last time I've checked it. The
docs doesn't mention any restrictions anymore. However, IMO a StringProperty()
would be fine too. Django itself has some magic to turn this into clickable
links.

http://codereview.appspot.com/2790041/diff/1/codereview/views.py
File codereview/views.py (right):

http://codereview.appspot.com/2790041/diff/1/codereview/views.py#newcode133
codereview/views.py:133: upstream = forms.URLField(required=False,
If we decide to store it as a StringProperty, this should be changed too.