Fix issue #90: Prevent user from joining or leading more than one hike at a time

[jsndz]

Description

This PR fixes the issue where users could join or lead more than one hike at a time.

Changes

• Added validation to prevent users from joining multiple beacons simultaneously.
• Prevented users from leading multiple hikes.

Fixes #90: Prevent user from joining or leading more than one hike at a time.

[Akshatji800]

@jsndz Why do we need this feature?

[jsndz]

prevents scheduling conflicts and overlapping responsibilities. And i think it is better to have this constraints so that the group's safety wont be compromised.

[Akshatji800]

Doesn't make much sense to me. I might need a solid reason with a real time usecase to get convinced.

[jsndz]

well if a user joins a hike at 9 am and the hike doesn't end till 1 pm he should not be allowed to join any other hike in between that time period since he can't be at 2 places