Rename the node and comment date tokens so they have proper namespaces

Currently the node created tokens are just things like [yyyy] and [since], and the node changed tokens are [mod-yyyy] or [mod-since]. Let's rename them to make it more clear (e.g. [node-created-yyyy] and [node-changed-yyyy]. We can also support the older style tokens during replacement, but just not provide the older ones in listings.