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.
bcd0733a64