Implement TPA RFC 30

TPA RFC 30 has been accepted, but not implemented in any of our sites. While working on lego#48 (closed), I added a new plugin that actually broke one of our sites, and decided to go around and implement this RFC everywhere so that adding a plugin to lego can't break a site.

Implementing this will mean:

• Removing the packages symlink
• Creating a new packages/ directory
• Symlinking plugins back in one at a time until the site works so that we're using only the necessary plugins

The plugins needed by this site are:

• i18n
• lektor-md-tag
• txt-to-html
• xml-to-html