Forbid HTML string tooltips (go-gitea#20935)

Tippy allows HTML strings to be passed as content but we do not use this
feature (we do pass HTML only as Element), so it's better to disable it
for increased security.

Ref: https://atomiks.github.io/tippyjs/v6/html-content/#string

web_src/js/modules/tippy.js

@@ -5,7 +5,7 @@ export function createTippy(target, opts = {}) {
     appendTo: document.body,
     placement: 'top-start',
     animation: false,
-    allowHTML: true,
+    allowHTML: false,
     maxWidth: 500, // increase over default 350px
     arrow: `<svg width="16" height="7"><path d="m0 7 8-7 8 7Z" class="tippy-svg-arrow-outer"/><path d="m0 8 8-7 8 7Z" class="tippy-svg-arrow-inner"/></svg>`,
     ...(opts?.role && {theme: opts.role}),