Login loop in code editor when no $.user object for given ID

[cpcallen]

A weird edge case which causes an unexpected login loop. To reproduce:

• Start a fresh database from the current core (as of 9a58103).
• Open a code editor for $.thing.name or any other unimportant property.
• Use your browser URL bar padlock menu to delete the ID cookie, if present.
• Make a trivial change to the property being edited (e.g. "The thing prototype").
• Click "Save".
• Note that a login window opens. Click "Sign In".
• Note that the code editor now has a butter bar saying "User not logged in", and the login window has reopened.

The cause is that, although the login server has successfully set the ID cookie, there is no $.user object corresponding to this ID in $.userDatabase.byMd5, because at the moment user objects are only created by the telnet server.

This has the effect of making it impossible to have code-only Code City instances. Proposed fix:

• Extract login and associated $.user-creation code to a general-purpose login function shared by both the telnet and http servers.
• Ensure that any save which results in the ownership of an object (e.g., because it creates a Function object) is enough to prevent the owning $.user object from being .destroyed. We don't want a user who's never "created a character" being assigned a band-new $.user object for every save!