Add team features

[yulmwu]

This PR includes the following changes to the Team (formerly known as Group) feature:

• If users are on the same team, solved challenges are now shared.
• And some refactoring

[codecov]

Codecov Report

❌ Patch coverage is 78.73462% with 121 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/http/handlers/handler.go	82.25%	27 Missing and 14 partials ⚠️
internal/service/team_service.go	65.00%	13 Missing and 8 partials ⚠️
internal/repo/submission_repo.go	73.61%	11 Missing and 8 partials ⚠️
internal/repo/team_repo.go	86.11%	5 Missing and 5 partials ⚠️
internal/http/router.go	0.00%	9 Missing ⚠️
internal/http/handlers/types.go	82.14%	5 Missing ⚠️
internal/service/auth_service.go	78.26%	3 Missing and 2 partials ⚠️
internal/service/ctf_service.go	50.00%	2 Missing and 3 partials ⚠️
internal/repo/user_repo.go	87.50%	2 Missing and 2 partials ⚠️
internal/service/helpers.go	84.61%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[yulmwu]

LGTM 78.73% Coverage