Support iOS Custom URL Scheme

[qrevel]

Is your feature request related to a problem? Please describe.
I'd like to open an iOS navigation application from umap with OSM metadata.
For example, opening direction to the nearest train station of a umap feature in Google Maps.

The custom URLs Schemes from Apple applications are currently not exposed through the Popup of a feature. The URL is completely stripped out.

Although Google Maps supports Universal Links, some other apps like navigation app like MagicEarth only supports URL Scheme.

Describe the solution you'd like
I believe it comes from the ALLOWED_URI_REGEXP that filters out custom URI.
My idea would be to open up that list to support any URI scheme.

Describe alternatives you've considered

Additional context
Might be linked to #1950 in order to have a fully customized HTML editor.
The current one is a "purified" version only.