Rework handling of transitions

Make header and footer comply with the Helmholtz CD guidelines.

• Fix wrong positioning of green slider in header
• Transitions have been redesigned to be a bit more flexible
• It is possible to specify which transition is intended while including
• Added Path in SVG to also allow right side slider if required

Cherry-picked from https://gitlab.hzdr.de/hifis/software.hifis.net/-/commit/29b969b4b3702814342051961ea7ed61e8e57905

changed the description

assigned to @erxleb87

@erxleb87 I cherry-picked your changes from https://gitlab.hzdr.de/hifis/software.hifis.net/-/commit/29b969b4b3702814342051961ea7ed61e8e57905. Would be nice, if you could double check that everything works as intended here as well and merge afterwars.

On my side everything seemed to work right.

I suppose the liquid including the transition could be a bit smoother. Also the now unused mixins can be removed.

After some consideration: Since we are aiming for a template anyways, I would not bother with it for now.

merged

mentioned in commit 3e7cff7a