msglist css: Fix inline border on text inside code blocks.

When the Markdown implementation on the server was upgraded in the
3.0-dev period, one subtle change was that code blocks started looking
like <pre><code>...</code></pre> instead of just <pre>...</pre>.

As a consequence, our CSS rules that were meant to apply to inline
code snippets started applying also to the text inside a code block.
Most conspicuously, we started showing not only a block-level border
around the whole code block (as intended), but also an inline border
around just the text inside the block.  That means a border starts on
the left of the text; runs above and below it to the end of just the
text on the line; then the border lines break and resume at the start
of the next line; etc., until wrapping around the right end of the
text on the last line.  We also showed a contrasting gray background
enclosed by that border, just as we do for inline code snippets,
giving the text in the code block a stripey appearance.

This fix is inspired by the webapp's changes that addressed the same
issue: zulip/zulip@43ac901ad and zulip/zulip@c00f626f8.

Fixes: zulip#4257

src/webview/static/base.css

@@ -39,6 +39,15 @@ code, pre {
   background-color: hsla(0, 0%, 50%, 0.125);
   font-family: Monaco, Menlo, Consolas, "Courier New", monospace;
 }
+pre code {
+  /* Starting with Zulip Server 3.0, code blocks have `pre > code`.
+     Undo the stuff we have for `code` that isn't meant to apply there. */
+  font-size: inherit;
+  white-space: inherit;
+  padding: 0;
+  border: none;
+  background: none;
+}
 table {
   border-collapse: collapse;
   width: 100%;