Set custom DOH

[ruihildt]

As a user, I want to set a custom DoH directly through the extension.

This is currently not possible to implement because the Firefox webextension API is not exposing this.

See:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1455425#c21
• https://bugzilla.mozilla.org/show_bug.cgi?id=1595859
• https://bugzilla.mozilla.org/show_bug.cgi?id=1455425
• https://bugzilla.mozilla.org/show_bug.cgi?id=1593788

[ruihildt]

See https://github.com/mullvad/browser-extension/issues/124

(In this case, if it becomes possible in the future, we can reopen the issue)