Fix various ImageDiff/SVG bugs (#23312)

Replace #23310, Close #19733

And fix various UI problems, including regressions from #22959 #22950
and more.

## SVG Detection

The old regexp may mismatch non-SVG files. This PR adds new tests for
those cases.

## UI Changes

### Before

![image](https://user-images.githubusercontent.com/2114189/222967716-f6ad8721-f46a-4a3f-9eb0-a89e488d3436.png)

![image](https://user-images.githubusercontent.com/2114189/222967780-8af8981a-e69d-4304-9dc4-0235582fa4f4.png)

### After

![image](https://user-images.githubusercontent.com/2114189/222967575-c21c23d4-0200-4e09-aac3-57895e853000.png)

![image](https://user-images.githubusercontent.com/2114189/222967585-8b8da262-bc96-441a-9851-8d3845f2659d.png)

![image](https://user-images.githubusercontent.com/2114189/222967595-58d9bea5-6df4-41fa-bf8a-86704117959d.png)

![image](https://user-images.githubusercontent.com/2114189/222967608-38757c1a-b8bd-4ebf-b7a8-3b30edb7f303.png)

![image](https://user-images.githubusercontent.com/2114189/222967623-9849a339-6fae-4484-8fa5-939e2fdacbf5.png)

![image](https://user-images.githubusercontent.com/2114189/222967633-4383d7dd-62ba-47a3-8c10-86f7ca7757ae.png)

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/features/imagediff.less

@@ -1,6 +1,6 @@
 .image-diff-container {
   text-align: center;
-  padding: 30px 0;
+  padding: 1em 0;
 
   img {
     border: 1px solid var(--color-primary-light-7);
@@ -22,6 +22,7 @@
       display: inline-block;
       line-height: 0;
       vertical-align: top;
+      margin: 0 1em;
 
       .side-header {
         font-weight: bold;
@@ -98,7 +99,7 @@
     }
 
     input {
-      width: 300px;
+      max-width: 300px;
     }
   }
 }