Remove fomantic loader module (#26670)

Replace Fomantic `loader` CSS module with our existing `is-loading`
spinner. Only three places in the UI used this module, which are
pictured here:

imagediff:
<img width="1237" alt="Screenshot 2023-08-22 at 22 18 01"
src="b0d82531-f05e-43c6-9e5b-1bfc268c056d">

webauthn:
<img width="894" alt="Screenshot 2023-08-22 at 22 05 05"
src="7b583425-d944-474a-a57a-22a65bbd8b29">

heatmap (I removed the previous loading text, it was unreadable because
it was tiny and on fast machines only visible for a fraction of a
second):
<img width="764" alt="Screenshot 2023-08-22 at 22 18 44"
src="1c7472d6-3e17-4224-a992-d8c0b380cc73">

Also, heatmap container does not resize any more after loading now and
previous duplicate id `user-heatmap` is gone.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/heatmap.js

@@ -30,8 +30,8 @@ export function initHeatmap() {
     };
 
     const View = createApp(ActivityHeatmap, {values, locale});
-
     View.mount(el);
+    el.classList.remove('is-loading');
   } catch (err) {
     console.error('Heatmap failed to load', err);
     el.textContent = 'Heatmap failed to load';