Overlapping problems on photos

[nmondollot]

Context

https://docs.google.com/document/d/11KSBD2S2Z6d9BQHD4uByzon4nEBVRYkmBmA2-mEitf0/edit#heading=h.y8qod8z94jla

Start groups

• compute start groups (code here and here)
• implement tap handling

Nb: we know show all the variants on the photos (we no longer hide them)

Other features

• sit start: add an icon and put the sit start info before the steepness info
• display a "missing line" message when line is missing
• variants badge:
  • the current problem should show up in the list (and count in the total when we display "xx variants")
  • make it easier to go back and forth between variants: the list should be exactly the same (same order)
    • see simplified logic