FEATURE: Use event category's color for calendar event

[nattsw]

We've hardcoded the events to var(--tertiary). Many users have requested that events be the colour of their categories.

I attempted to add an acceptance test to assert that the colours match the category colours defined in the fixtures, but

query(".fc-event-container").style.backgroundColor

returns 'rgb(37, 170, 226)' value which is slightly annoying to assert against...

Category calendar now:

Upcoming calendar now:

[tgxworld]

returns 'rgb(37, 170, 226)' value which is slightly annoying to assert against...

I think this is fine given that for a given hex value we can only get a certain rgb value.