Fix duplicate sub-path for avatars (#31365)

Fix #31361, and add tests

And this PR introduces an undocumented & debug-purpose-only config
option: `USE_SUB_URL_PATH`. It does nothing for end users, it only helps
the development of sub-path related problems.

And also fix #31366

Co-authored-by: @ExplodingDragon

routers/common/middleware_test.go

@@ -61,7 +61,7 @@ func TestStripSlashesMiddleware(t *testing.T) {
 		})
 
 		// pass the test middleware to validate the changes
-		handlerToTest := stripSlashesMiddleware(testMiddleware)
+		handlerToTest := normalizeRequestPathMiddleware(testMiddleware)
 		// create a mock request to use
 		req := httptest.NewRequest("GET", tt.inputPath, nil)
 		// call the handler using a mock response recorder