main/postmarketos-ui-phosh: open links in firefox

Add mimeapps.list, so links clicked in postmarketos-welcome and other applications properly open in firefox. Without this patch, they just don't open at all.

Bump the pkgver instead of the pkgrel, so the APKBUILD is in sync again on both master and v20.05 (right now, the pkgrel on v20.05 is ahead by one, otherwise the file is the same).

Related: https://help.gnome.org/admin/system-admin-guide/stable/mime-types-application.html.en

I've verified, that the patch works on the PinePhone.