Check that the reactions `set_from_email` `redirect_url` is legit

[issyl0]

• These email reactions URLs look like https://soulmedicine.io/pathways/[...]/reaction/set_from_email?reaction_name=[...]&redirect_url=https://soulmedicine.io[...].
• Fixes https://github.com/chaynHQ/soulmedicine/security/code-scanning/1.