Fix URL related escaping for oauth2 (#37334)

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.

services/context/base_path.go

@@ -44,9 +44,9 @@ func (b *Base) PathParamInt(p string) int {
 
 // SetPathParam set request path params into routes
 func (b *Base) SetPathParam(name, value string) {
-	if strings.HasPrefix(name, ":") {
-		setting.PanicInDevOrTesting("path param should not start with ':'")
-		name = name[1:]
-	}
 	chi.RouteContext(b).URLParams.Add(name, url.PathEscape(value))
 }
+
+func (b *Base) SetPathParamRaw(name, value string) {
+	chi.RouteContext(b).URLParams.Add(name, value)
+}