Follow up #37327. See the comments.
* Root problem: the design of OAuth2 providers is a mess, the display
name is used as provider's name and used in the URL directly
* The regressions:
* When trying to fix https://github.com/go-gitea/gitea/issues/36409 , it
introduced inconsistent URL escaping for the "path" part.
* This fix: always use "path escaping" for the path part, add more tests
to cover all escaping cases.
Now, frontend "pathEscape" and "pathEscapeSegments" generate exactly the
same result as backend.
aee6628bf5