Use other contest as a template

[LightFelicis]

Context:
Polish teachers that are using Szkopuł host 10+ groups (contests) each year. Since we created Szkopuł Kursy, they manually copy same tasks for each contest (because they want separate rankings and adjusted tasks per class). This is a huge toil for them and multiple teachers requested me to implement a "copy contest" option.

Goals:

• When creating contest, user can import all tasks and rounds from other contest in which they are a teacher.
• Teacher can also copy from public contest (this way we can develop Szkopuł Kursy further)

Why it's important:
Adoption of Szkopuł is growing and we should align with common expectations and adress pain points.

[LightFelicis]

@twalen Could you please help with triaging this? Is it on TAG execution backlog for this semester?
This would greatly benefit the users.

[twalen]

Classes resume next week. I will discuss this ticket on next Wed.