implement tank management page

[cdent]

This would be a page where you can:

• [x] edit the tank's policy
• [x] edit the tank's description
• [ ] get mass view on all the tiddlers
• [ ] do mass delete of some selection of tiddlers
• [ ] do mass apply tag to some selection of tiddlers

A first pass probably wouldn't bother paginating, but eventually such a thing would be good.

A first pass would probably just be for one tank. A second pass might have each tank in a tab and allow dragging tiddlers to a different tab?

/me needs help and a designer

[cdent]

This may override #19

[cdent]

I don't think it does override #19, see EntityManagement.