Check required attributes

[orhoj]

Purpose

Check that attributes marked as required by the schema are available in the credential being added to the wallet.

Changes

• Check the required attributes on the add page.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.