support direct link to exercise

[daveho]

It should be possible to link directly to an exercise.

One idea is to use an anchor (#foo/bar/baz) on the URL,
have the webapp check for this, and navigate automatically to
the development mode to work on that exercise (once the
user has logged in.)

[daveho]

I've made some progress on this. The url-page-links
branch in my repository has an initial proof of concept that
works for links to exercises.

The next step is to generalize this by supporting links
to all internally-navigable pages. Here is the idea:
each CloudCoderPage has a set of objects that need
to be in the session in order for the page to operate.
We make these an explicit prerequisite for activating
the page: for example, the DevelopmentPage needs
the CourseSelection and Problem. The URL fragment
in the page link can specify the ids of these objects
so that they can be loaded in a generic fashion when
there is a direct link to the page.

When a page is reached via internal navigation, it should
set the URL fragment to indicate the ids of the objects
it needs, so that the user can copy the URL and
navigate back to that page (or post it for others to follow).

As a side benefit, this will allow the overall CloudCoder
UI to work nicely with the browser forward and back
button, since the URL will provide adequate context to
get the user back to where she was.

[daveho]

More progress - my url-page-links branch has a general mechanism
for loading page objects based on parameters in the URL
fragment. Works for DevelopmentPage - needs to be
implemented for the other pages.

[daveho]

Page object loading should now work for all pages. Merged
onto master branch, so I'm closing the issue.