[Form] allow template to set the onsubmit tag

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -

This change could be backported to the Zikula 1.3.x branch too, but I don't know how to do it (is there a special workflow or should I make just another PR?)