[super] Improve wiki-style functionality for history

[thejimmyg]

At the moment we have a good revisioning system but a poor history interface. We need to improve this in a number of areas:

• #191 Searching by modification date
• #193 Searching by time-related field
• #301 Package discussion pages
• Package history page should provide links to pages at particular revisions, similar to the wikipedia pages
• Viewing old revisions or unmoderated changes should have a message at the top of the package page
• Other improvements as per my word doc.