Prevent modification of CSP header when Invidious companion is present

[Fijxu]

This piece of code can be set just one time in a constant instead of calculating it again and again on each /watch and /embed request.

https://github.com/iv-org/invidious/blob/42d34cd08484fc9ddb8b53e4cdf0a26bda01ca54/src/invidious/routes/embed.cr#L212-L222