Checkboxes fix

[NicoHood]

• Fix Multiple issues with checkboxes #471 checkboxes state when all boxes are empty
• Fixed checkboxes translation in email template

[NicoHood]

Would you mind merging this?

[rhukster]

I will look at this one again now I've released Form.

[NicoHood]

I have rebased my changes, simply removing the translation fix, as it was merged in #494

[mahagr]

I won't accept this fix as it won't work in admin (it doesn't use form plugin, just the field), but I have an idea how to fix this.

[mahagr]

I will try another approach.

[NicoHood]

Any progress on this issue?