Move form event binding to coffee/.js.
Add some input validation to price tag form price.
Fix price checks on price update and item add.

- There were not a whole lot of checks for these yet, but they should now
  work as expected.
Adds way to change modes from menu

* Menu can be enabled and disabled via switcher.
* Changed menu to be dropdown.
* Menu items have now "descriptive" icons.
* Using the menu, current mode can be changed.
Changed createRow to be a bit more robust with price-variable

Price formatted as price only if it is number. Otherwise, rounding is
disabled and the value (or blank default) is displayed as-is.
Adds success-class for rows generated in finish receipt.
Adds support to abort receipt

* Items are released and the receipt ended.
Adds static Clerk registration for admin.
A bit more validation for price

* Price has-error if the parsed result is not a number.
* Catch Decimal processing exception on invalid input.
* Removed debug(?) output.