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.
In history (from some legacy frameworks), both `:name` and `name` are
supported as path path name, `:name` is an alias to `name`.
To make code consistent, now we should only use `name` but not `:name`.
Also added panic check in related functions to make sure the name won't
be abused in case some downstreams still use them.
