HTML5 tweaks/improvements. Closes #1408

* Add support for min/max/step attributes for int and float settings in the admin center
* Add support for min/max/step attributes for "number" theme settings and use the "number" input type for these settings where appropriate
* Improve default time offset setting in the admin center - adds min and max values and changes the step to 0.25 to support the two weird timezones

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>