Added tag management interface.

Added:
    - Tag list.
    - Tag detail page.
    - Tags can now have a category.
Resolved bug with changing match string of a tag rule.
More tags administration.

Added:
    - Interface to tags.
    - Interface to auto tag entries.

Changed:
    - Tags can now be assigned a category, but currently only from the
      tag administration interface.
    - Tags have a hidden boolean attribute "auto_created" to distinguish
      automatically added tags from user added ones.
Don't show autotags in tags list.
Only show autotagged entries in the list created by autotagging.