Change "mailto:" urls to be external

[BPerlakiH]

Fixes: #780

[kelson42]

@BPerlakiH Is that the right logic!? Everything which is not zim:// should be external IMHO.

We have a few other schemes, for example, tel: geo: ftp:...

[BPerlakiH]

@BPerlakiH Is that the right logic!? Everything which is not zim:// should be external IMHO.

We have a few other schemes, for example, tel: geo: ftp:...

I did found that we handle "geo" scheme separately. Should we say that everything apart from "kiwix" and "geo" is external then ?

[BPerlakiH]

@kelson42 Please see it updated.

[kelson42]

@BPerlakiH This looks more what I was expacted. From a user perspective, its important the OS tries to open the link with the appropriate app, in the case of mailto: the default mail app.

Here a few remarks which are related to the naming of things:

• We should have a zim:// protocol, not a kiwix:// protocol, can we change that in a dedicated issue/PR?
• I challenge the "external" naming, probably "unsupported" would be better?

[BPerlakiH]

The "mailto:" is opening the mail app, I've tested that (on macOS).
I've opened an issue for the url scheme change: #798
Changed the naming from "isExternal" to "isUnsupported".