Add more advanced admin tools

[meshy]

The admin is rather rudimentary as of #115. We could make it more powerful with a few additions:

• "Add child" link on each element of the list view.
• "Move with children" link for each route.
• "Swap with" link (move children optional).
• Change "url" form-field to use a single-line widget (Block newline chars in Route.url by changing widget #85).

Some of these are probably best tackled after #74 has been completed, as the addition of a Site to each Route will surely complicate things.