ADX-1041 Support for custom flash messages passed from ADR

[tomeksabala]

Description

1. Fixed a bug where success flash message and return to after_save_url happened if any flash messages present
2. Added support for passing a custom flash message on initial request by url arg flash_message.

Checklist

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• The Jira ticket for this issue has been updated to "Ready to Review" or equivalent.
• I have developed these changes in discussion with the appropriate project manager.
• My code follows the general Fjelltopp documentation (see Confluence).
• I have made corresponding changes to the Fjelltopp documentation (see Confluence).
• I have rebased this branch with master.
• New dependency changes have been committed.
• I have added automated tests that prove my fix is effective or that my feature works.
• New and existing tests pass locally with my changes.
• My changes generate no new warnings.
• I have performed a self-review of my own code.
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