Add credential metadata fallback

[orhoj]

Purpose

Handle credential metadata being changed (in the contract) to something invalid.

Changes

• Save the metadata URL that is received when adding the credential, and use this if the credential entry (or the temp metadata url) does not match anything in storage.

Note that we should also be updating the fallback URL everytime we get new valid metadata, but we can add that in the future.

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.