Added user list page at _users

This displays a table of all the users of the site, including the times they last logged in.

By default, the table does not show user's email addresses. I added a config option that enables this which is off by default. When displaying email addresses, it uses pandoc's reference obfuscation.

I also added two fields, both `UTCTime`, to the `User` data type: `uCreated` and `uLastSeen`. In order for this to be backwards-compatible with existing installs of gitit, I added a hand-written `Read` instance for `User`, which can parse the old `User` type as well as the new one.