Improve Django Admin for efficient review

[pombredanne]

There are likely a few tweak to the admin that could go a long way to help until we have a GUI #24

[tdruez]

Some of the recent Admin UI improvements:

• Make column values filters link for admin changelist
• Add ability to filter by path in CodebaseResource admin
• Add CodebaseResourceAdmin raw view to download and view file content
• Add links to resources and packages in Admin changelist
• Display license_expressions as a joined list in the Admin

Closing. We'll open new tickets for specific admin features.