IDEA GATHERING: Field validation for drafts and posting idea (sub-issue of #1786)

[BeccaMelhuish]

As discussed in PM and here #1957 we need to sort out the field validation for add your idea field.

Let's do it like this, as @DiverVM suggested:

• Both 'Save as draft' and 'Add you idea' buttons are always active
• When user clicks 'Save as draft', we check that the idea heading is filled in. If it is isn't, this triggers the 'Field is mandatory' error.
• When user clicks 'Add you idea', we check that all fields are filled in. If any of them aren't, this triggers the 'Field is mandatory' error for that/those fields.

Side note if relevant: 'Field is mandatory' is fine as the error message for all such fields (The '0 to X characters' is a bit unexpected/confusing anyway).

Marking as a sub-issue of #1786 draft ideas and putting to in progress for @DiverVM. Should be fixed before we release drafts :)