content: Show spinner for thumbnail placeholders

This is a port of zulip/zulip#30477.  That PR isn't yet merged, but
it's been deployed on chat.zulip.org and I believe the styling is
unlikely to change.  (If it does get tweaked, we can always update.)

The placeholder `img` elements already come with a `src` pointing
to this same `loader-black.svg` file, but on the Zulip server.
So the effects of this change are:

 * The placeholder image is local instead of remote, which may
   improve the experience a bit on a slow connection.

 * In dark theme, we use an appropriately contrasting white spinner,
   instead of a black spinner that blends in with the background.

Fixes: zulip#5875

src/webview/css/cssNight.js

@@ -17,4 +17,7 @@ body {
 .highlight {
   background-color: hsla(51, 100%, 64%, 0.42);
 }
+.message_inline_image img.image-loading-placeholder {
+  content: url("images/loader-white.svg");
+}
 `;

src/webview/static/base.css

@@ -552,6 +552,9 @@ time {
   height: 160px;
   object-fit: scale-down;
 }
+.message_inline_image img.image-loading-placeholder {
+  content: url("images/loader-black.svg");
+}
 
 .message_inline_video {
   position: relative;

tools/build-webview

@@ -195,6 +195,8 @@ sync "src/webview/static" "${dest}" <<EOF
 + /base.css
 + /images/
 + /images/follow.svg
++ /images/loader-black.svg
++ /images/loader-white.svg
 + /images/play_button.svg
 - *
 EOF