Cleanup/refactor time-zone offset preference and relative date

Cleanup/refactor time-zone offset preference and relative date
preference handling.

Time-zone offsets: the allowable range has to be at least [-24,24]
hours (maybe a bit more, depending on daylight savings, etc...)
Also note that there are some 1/4 hour time zones.
I think its really much cleaner just to use a text input to enter
the time zone offset.

Relative Dates:  Unified code for generating "Last edited on" message.
(This logic was replicated in various places, and was buggy...)

Theme:
 formatDateTime() now always gives you a Date and Time.
 formatDate() now always gives you a Date.
 use getDay() if you want a (possibly) relative date.
 use getLastModifiedMessage() to get the "Last edited on" message.

These modifications slightly alter the markup of the last edited
dates in the Portland theme, and the stardates in the WikiTrek theme.
(But I think the gains in code reuse and simplicity are wroth the
sacrifice.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1942 96ab9672-09ca-45d6-a79d-3d69d39ca109