Improve templates.

- Convert from text/template to html/template.
- Comppose templates from site-wide skelton and page specific head and
  body.
Record more package information.

- Packages mentioned in readme files.
- Test imports.
- Version control system.
Add fork detection and removal to admin tool.

Add new subcommand 'prune' to gddo-admin. This subcommand looks for and
removes forks. The subcommand also removes subtrees where all packages
have search rank = 0.
Firt pass at supporting presentations.