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="https://github.com/go-gitea/gitea/assets/115237/b0d82531-f05e-43c6-9e5b-1bfc268c056d">

webauthn:
<img width="894" alt="Screenshot 2023-08-22 at 22 05 05"
src="https://github.com/go-gitea/gitea/assets/115237/7b583425-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="https://github.com/go-gitea/gitea/assets/115237/1c7472d6-3e17-4224-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/components/ActivityHeatmap.vue

@@ -1,18 +1,16 @@
 <template>
-  <div id="user-heatmap">
-    <div class="total-contributions">
-      {{ locale.contributions_in_the_last_12_months }}
-    </div>
-    <calendar-heatmap
-      :locale="locale"
-      :no-data-text="locale.no_contributions"
-      :tooltip-unit="locale.contributions"
-      :end-date="endDate"
-      :values="values"
-      :range-color="colorRange"
-      @day-click="handleDayClick($event)"
-    />
+  <div class="total-contributions">
+    {{ locale.contributions_in_the_last_12_months }}
   </div>
+  <calendar-heatmap
+    :locale="locale"
+    :no-data-text="locale.no_contributions"
+    :tooltip-unit="locale.contributions"
+    :end-date="endDate"
+    :values="values"
+    :range-color="colorRange"
+    @day-click="handleDayClick($event)"
+  />
 </template>
 <script>
 import {CalendarHeatmap} from 'vue3-calendar-heatmap';

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';