normalize double slash in SENTRY_DSN to avoid 403 CSRF error

[lambdaq]

I am using the latest on-premise docker image of sentry

in sentry/models/projectkey.py

the return '%s%s?sentry_key=%s' syntax has a serious potential bug.

If in /manage/settings -> General -> Root URL we have an ending tail slash, then at the page

XXX/XXX_proj/settings/keys/ the DSN, CSP endpoint would have duplicate double slash

Others have reported the bug ealier https://forum.sentry.io/t/csrf-verification-failed/2901

We should normalize the DSN URL.

Beside, on 403-csrf-falure.html we should add extra notes for this behavior.

[fungiboletus]

I think this could be a very good improvement, as I just lost time thanks to a stupid tail slash.

[ThoTischner]

Please fix =)

[vaIgarashi]

Bump

[BYK]

I think this has the same root cause with #8657. Closing in favor of that.