flatpak: Use the ".Devel" suffix instead of "Devel" for the nightly version

Fixes #3141 (closed)

I hope I did it correctly, and that I did not miss anything. Just flying with instruments here.

Things I changed:

$ git grep "\.Devel"

.gitlab-ci.yml:        APP_ID: "org.gnome.Nautilus.Devel"
.gitlab/issue_templates/Bug.md:    flatpak install --from https://nightly.gnome.org/repo/appstream/org.gnome.Nautilus.Devel.flatpakref
.gitlab/issue_templates/Bug.md:    3) The Nightly version can now be launched from Activities, or with this command: flatpak run org.gnome.Nautilus.Devel
.gitlab/issue_templates/Shortcoming.md:    flatpak install --from https://nightly.gnome.org/repo/appstream/org.gnome.Nautilus.Devel.flatpakref
.gitlab/issue_templates/Shortcoming.md:    3) The Nightly version can now be launched from Activities, or with this command: flatpak run org.gnome.Nautilus.Devel
build-aux/flatpak/org.gnome.Nautilus.json:  "app-id": "org.gnome.Nautilus.Devel",
build-aux/flatpak/org.gnome.Nautilus.json:    "--talk-name=org.gnome.NautilusPreviewer.Devel",
src/nautilus-previewer.c:    else if (g_strcmp0 (previewer_dbus_name, "org.gnome.NautilusPreviewer.Devel") == 0)
src/nautilus-previewer.c:    else if (g_strcmp0 (previewer_dbus_name, "org.gnome.NautilusPreviewer.Devel") == 0)

Things I left alone as I presumed they are unrelated:

$  git grep -i "devel" | grep -vi develop | grep -v "\.Devel"

.gitlab-ci.yml:.vars-devel:
.gitlab-ci.yml:    extends: ['.flatpak@x86_64', '.vars-devel']
.gitlab-ci.yml:    extends: ['.flatpak@aarch64', '.vars-devel']
.gitlab/Dockerfile: && dnf install -y appstream-devel \
build-aux/flatpak/org.gnome.Nautilus.json:    "devel",
build-aux/flatpak/org.gnome.Nautilus.json:        "-Dprofile=Devel",
meson.build:if get_option('profile') == 'Devel'
meson.build:    fallback: profile != '' ? 'devel' : '',
src/nautilus-application.c:        gtk_widget_add_css_class (GTK_WIDGET (window), "devel");