restructure forms

[raffaelj]

The structure grew over time with multiple changes in the Multiplane files and in the FormValidation addon. At first, I didn't think about the possibility to have multiple forms on the same page. I only had types of forms in mind (contact form, newsletter subscription, comment section)...

to do

• move html helper functions to a helper class
• add js to use the form with ajax requests and client side validation (it still must work without js!)
• all form parts should be in one location (or in less, clearly arranged locations), maybe as own module/addon
  • partials in rljbase theme
    • /widgets/contactform.php
    • /partials/form.php
    • /partials/formfields/*.php
  • /module/forms.php
  • /Controller/Forms.php
• move some php from partials to controller/helper
• improve multilingual forms
  • all form labels and info texts are translated via config i18n file
  • form field contents should be translatable, too
  • translations should be in FormValidation addon instead of i18n file, so it doesn't break when users change the forms
• document, how to use the FormValidation addon with CpMultiplane