views/schild: Form design rework

Rather large refactoring:
- A bunch of CSS so that it doesn’t look like ass;
- Roll our own (WT)Forms, as wtforms-sqlalchemy is too limited.  Drop that one
  completely;
- Deduplicate code from /schild/<ident> and /create;
- As SchildForm.{image, template} are now RadioFields, iterate directly via
  their .choices property instead of passing templates/images separately to the
  template;
- … and some minor shenanigans as usual.