Set colors per stage

[Goosly]

A competition may have multiple stages. A stage may have a color.

If colors are assigned to stages, this affects the following:

• Scorecards are separated by stage, so they can be printed on sheets of the correct color (see this issue )
• The personal schedule includes the stage color as a background color for the assignment
• The table overview includes the stage color as a background color for the assignment

[KitClement]

Can I suggest that for black and white printing that the option to display the group with a letter-number combo be possible? E.g. For group one on the blue stage, have it show up as "B1" rather than use background colors. If it's simpler trnaslation-wise, I'd also be fine with having stages be alphabetically labeled and have groups A1, B1, C1, A2, B2, C2, etc.

[Goosly]

Partially implemented, needs a lot of testing though