Fixed CHOT-81 and CHOT-83
"Edit" and "Delete" butons are inactive, when car_class field is empty
Now we can see age bounds of competition on "Edit racer page".
Value "0" now is invalid in "Edit number" form
Fixed CHOT-80, 81, 83
We can change car number 0
Field 'Car class' not determined by the age of the participant (from -
to) on the 'Edit page'
Activity 'Edit number' button
'Delete all' button is also disabled when 'car_class' field is empty
Fixed activity of 'Delete' button at add racer page
Merge branch 'mgalctc_80_81_83'