Several backend / frontend fixes

[dukeofharen]

Pull request type

Please check the type of change your PR introduces:

• [X] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [X] 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?

• Fixed small bug where form helper would not close correctly in "Add request / response value" menu.
• Fixed small bug in reverse proxy response writer where root URL would be replaced incorrectly.
• When using the reverse proxy response writer, take a look at the response and determine if it is binary or not.
• Added stub validation on response headers.
• Added configuration value for "allowGlobalFileSearch" which enables or disables whether users can use the "file response writer" to look for files in the whole OS or only for files relative to the stub .yml files or the file storage location.

Does this introduce a breaking change?

• [ ] Yes
• [X] No