Proposal: Random color for new category

[xavier-calland]

When adding a new category or when importing from Conferance-Hall, it would be great to have different colors.

Current behavior seem to be :

• new category from "+" button in "Event settings" : color is back
• all categories from Conferall-Hall : color is DEFAULT_SESSION_CARD_BACKGROUND_COLOR

Proposal When creating a new category, generate and assign a color to the category

[HugoGresse]

Good idea, seems pretty straight forward todo

[HugoGresse]

Done and deployed!