Several small fixes

[dukeofharen]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

• Added date/time to stubs .yml after download.
• Increased the character count where the CodeMirror editor automatically switches to the simple editor from 2000 to 4000.
• Added extra logic to JSON condition checker so boolean values can now also be checked by a regex.
• Added CSS spinner to show when asynchronous (HTTP) calls are working.
• Show validation error when stub response text is set and HTTP status code is 204.
• Added fix so it is now possible to also send YAML to the API which contains fields that are not in the schema.

Does this introduce a breaking change?

• Yes
• No

Other information