pmos-ui-{gnome,phosh}: refactor pmb_recommends

This is mostly a rebase of !3700 (closed), and it does three things:

1. moves a lot of shared pmb_recommends packages from the gnome-mobile and phosh meta UI packages to the postmarketos-base-ui-gnome package. Note that the packages that were kept are specific to mobile UIs and not applicable to the full gnome desktop, or there are better alternatives for the full desktop.

2. implicitly adds a bunch of useful apps to the gnome meta UI package so that they are available by default on new installations. Some might be a little opinionated, but I think most folks installing this UI would expect most or all of these to be available by default so it felt worth adding them.

3. install flatpak and set up the flathub repo (new installs only)

This requires a recent (currently only from master branch?) pmbootstrap, that can support pmb_recommends in arbitrary packages.