Delete unused code.

Delete the unused code for displaying hyper-linked source code for the
standard packages. The data file generator for the feature no longer
works because the generator was written to an old version of go/types.
If the feature is restored, then it should be written from scratch to
take advantage of oracle features.
Add flash messages.

Add support for displaying one-time notification messages on package
pages. This type of message is commonly called a "flash message."

Display a flash message when redirecting to the canonical path for a
package.

Change the package refresh handler to redirect back to the package page
with a flash message on error.