[journeys] implementation

The intermediate element journey was created. One can add multiple journeys to a gesture.

• edit journeys from the custom gesture editing

We still need to:

• visualize journeys in index (done but not online yet)

composition

• make journeys' shards available in the composition mode

display

• display journeys in red/pink on the shards of the homepage (same style as for gestures' space but other color) display journeys on :
• the gesture's map (exemple: https://eavatea.jubilee-art.org/gesture/67?map=show)
  • fix the gesture's maps in general (je pense que c'est cassé ou alors c'est juste lent) -- edit c'est juste très très lent je pense
• delete shard from composition
• the markers' map if the journey is on the same geographical zone (thick pink line, as before) when we click on a journey's shard (in a gesture page) :
• display a map on the left side - similar to what we have for markers now (we had the idea to put the first point of the journey as the center of the map)
• display a journey's panel on the right side, with name and description (@julie.v )