ci: update to use the CI Template

Closes #5

See merge request GNOME/gnome-tour!4

https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak

Close #5

fixes #2

Minor tweaks

See merge request bilelmoussaoui/gnome-tour!2

assets: proper unified scale

See merge request bilelmoussaoui/gnome-tour!1

- scaled up all assets for them to render sharp
- unified the sizing so they don't jump around
- unified stroke sizes
- optimized with SVGO

original assets to create derivatives are at
https://gitlab.gnome.org/Teams/Design/os-mockups/tree/master/greeter/assets