css: fix conversation width in the HTML element

> Add min-width:0 to .conversation-chat in shim.css. This is not only a
> fix, but the correct fix.
>
> But WTF?
>
> Well quite simply, the default value of min-width is auto, which
> normally acts like 0, but for a flex item, auto becomes something more
> along the lines of min-content (except special complicated rules that I
> don't fully understand apply). The net result is that the minimum width
> prevents the code boxes in the conversation from using their overflow:
> auto to enable scrolling. Setting min-width: 0 is effectively just
> turning this special rule off, and causing it use more normal box sizing
> rules.
>
> I hate how weird and impossible to understand the css box model is.

Signed-off-by: Xe Iaso <me@christine.website>

css/shim.css

@@ -36,6 +36,7 @@ img {
 
 .conversation-chat {
     align-self: center;
+    min-width: 0;
 }
 
 .gruvbox-dark pre, pre {