Add apple-touch-icon for ipad webapp

[el]

After the release of the new iPadOS, code-server works pretty well on the iPad.

Configuring the web app for the ipad makes sense now:

https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html

Here is how it looks right now:

[Revengervn]

Also, another important point:
Version: MacOSX binary 64 bit.
Coder's copy,cut and paste doesn't work on the ipadOS safari which is weird.
Theia and other VS code-based IDE work perfectly.
I spent the entire day trying to figure out why but couldn't.

[code-asher]

Was added by #1060.