Make a user summary page

[jace]

Kharcha needs a /<workspace>/users/<user> page that shows a summary of the user's reports within the workspace.

[jace]

To implement this, the <user> bit must be either a userid, which is ugly but straightforward, or a username falling back to a userid, which requires new functionality in coaster.views.load_models. It's not capable of advanced filters at the moment.

[jace]

As discussed in coaster's #17