Removed hand-baked type coercion in form system and moved to using click.types. This means coercion stuff only has to be implemented once and works for forms as well as the CLI. A good deal of cleanup in the form system is still needed to depopulate form/fields.py a bit.

Oh also, the certificate generated by the `minica` command of the CLI now properly works with <keygen> and a good bunch of assorted bugs was fixed. Probably while introducing some new ones.