feat: add "Try It" button for /apps

[WofWca]

Hidden by default for now.

See https://support.delta.chat/t/run-webxdc-apps-in-the-browser-we-added-try-it-button-to-the-app-store/4445?u=wofwca.

I am committed to maintaining the https://webxdc-try-it.xyz site
for at least 2 years.
However, if this feature is liked it would be really nice
to do the deployment on some more "official" servers,
such as webxdc.org itself.
Luckily, the deployment isn't that hard.
See https://github.com/DavidSM10/web-app-runner?tab=readme-ov-file#how-set-it-up-online.

The reason we can't simply always show this button
is because the the /apps page is used by Delta Chat for Android
as the app picker ("Add Attachment"), where such a button doesn't make sense.
Also maybe Delta Touch does the same.

I suggest to act as follows:

1. Merge this.
2. On Delta Chat Android (and possibly Delta Touch (and iOS)),
   add a query (search) param hideTryItButton to the app picker's URL,
   so that the website's JS can read it and disable the "Try It" button.
3. Wait for sufficient deployment of the new Delta Chat version
   from the previous steps.
4. Enable the "Try It" button by default.
5. (later) deploy the "Try It" website on a webxdc.org subdomain,
   so that you don't have to hope that I don't get hit by a bus.

CC @davidsm10