Better handling of invalid origins in the config

[lazka]

We just had one user adding https://foo.bar.org/ into the config, which didn't work because of the trailing slash and the origin being https://foo.bar.org.

We should either:

• Validate the input somehow and show an error to the user, to make sure the origins are all in the correct format
• Or allow this specific case and strip trailing slashes

(I kinda prefer (1) because it covers more cases)

Somewhat related, I wonder if we support explicit ports like http://foo.bar.org:80 in that case ?