Rework and fix stopwatch (#30732)

Fixes https://github.com/go-gitea/gitea/issues/30721 and overhauls the
stopwatch. Time is now shown inside the "dot" icon and on both mobile
and desktop. All rendering is now done by `<relative-time>`, the
`pretty-ms` dependency is dropped.

Desktop:
<img width="557" alt="Screenshot 2024-04-29 at 22 33 27"
src="3a46cdbf-6af2-4bf9-b07f-021348badaac">

Mobile:
<img width="640" alt="Screenshot 2024-04-29 at 22 34 19"
src="8a2beea7-bd5d-473f-8fff-66f63fd50877">

Note for tippy:
Previously, tippy instances defaulted to "menu" theme, but that theme is
really only meant for `.ui.menu`, so it was not optimal for the
stopwatch popover.

This introduces a unopinionated `default` theme that has no padding and
should be suitable for all content. I reviewed all existing uses and
explicitely set the desired `theme` on all of them.

web_src/js/features/repo-issue.js

@@ -502,6 +502,7 @@ export function initRepoPullRequestReview() {
   if ($reviewBtn.length && $panel.length) {
     const tippy = createTippy($reviewBtn[0], {
       content: $panel[0],
+      theme: 'default',
       placement: 'bottom',
       trigger: 'click',
       maxWidth: 'none',