Only show one tippy at a time (#24648)

Because our tippy instances have an `interactiveBorder`, it's possible
to bring up two instances at once, which is undesirable.

<img width="256" alt="Screenshot 2023-05-10 at 23 03 04"
src="https://github.com/go-gitea/gitea/assets/115237/3a9a1775-78c1-46d4-a8a5-503ab7dca0d8">

Prevent this by keeping track of visible tippy instances and hiding
others when one is shown. Tippy also has the [singleton
addon](https://atomiks.github.io/tippyjs/v6/addons/#singleton) for the
same purpose, but it's unsuitable to us because we don't init all
tooltips at once.

web_src/js/modules/tippy.js

@@ -1,5 +1,7 @@
 import tippy from 'tippy.js';
 
+const visibleInstances = new Set();
+
 export function createTippy(target, opts = {}) {
   const instance = tippy(target, {
     appendTo: document.body,
@@ -9,6 +11,18 @@ export function createTippy(target, opts = {}) {
     interactiveBorder: 20,
     ignoreAttributes: true,
     maxWidth: 500, // increase over default 350px
+    onHide: (instance) => {
+      visibleInstances.delete(instance);
+    },
+    onDestroy: (instance) => {
+      visibleInstances.delete(instance);
+    },
+    onShow: (instance) => {
+      for (const visibleInstance of visibleInstances) {
+        visibleInstance.hide(); // hide other instances
+      }
+      visibleInstances.add(instance);
+    },
     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}),
     ...opts,