Coverts a function into an action that can be rendered into HTML. A
unique, hard to guess string is generated for the function, and a
function is added to the session hashtable under this string. The
string is then returned. When later requests come in,
'get-request-action' machinery determines if the action string that
came with the request is stored in the hashtable, and if so, invokes
the stored function.

'action-fn' - A function of zero arguments that will be called if
the user initiates appropriate control (link, form, etc.)

'action-code' - The code to use for an action (if not specified
make-action generates a unique value for each action). Note, if you
don't provide a hard to guess code ('generate-action-code' is used by
default), the user will be vulnerable to an attack where a malicious
attacker can attempt to guess a dangerour action id and send the user
a link to it. Only use guessable action codes for GET actions.