$wgMobileFrontendLogo is used for a standalone `icon`. It should make use of `wgLogos`
Soon as part of T246170 we will be adding an `icon` field pointing to the project standalone logo. and will be converted to an SVG
In preparation for that I suggest we replace usages of $wgMobileFrontendLogo with `ResourceLoaderSkinModule::getAvailableLogos( $config )['icon']`
= Acceptance criteria
[x] Update config to add `icon` field to wgLogos and to map to $wgMobileFrontendLogo
[x] Update MobileFrontend to use wgLogos
[ ] Drop $wgMobileFrontendLogo config (https://gerrit.wikimedia.org/r/c/584734)
= QA
[] Visit login page on https://en.m.wikipedia.beta.wmflabs.org/wiki/Special:UserLogin
[] Confirm it shows a W logo.
[] Visit https://en.m.wikivoyage.beta.wmflabs.org/wiki/Special:UserLogin and confirm logo matches the wikivoyage logo.
[] Visit https://en.m.wikisource.beta.wmflabs.org/wiki/Special:UserLogin and confirm logo matches the wikisource logo
[] Visit https://commons.m.wikimedia.beta.wmflabs.org/w/index.php?title=Special:UserLogin&returnto=Main+Page&returntoquery=mobileaction%3Dtoggle_view_mobile and confirm the commons logo matches.