Registering a user to the spawn island doesn't change its status as a spawn island

[tastybento]

Description

Describe the bug

Registering a user to the spawn island doesn't change its status as a spawn island.

Steps to reproduce the behavior

1. Set an island to be spawn: /bsb setspawn
2. Do /bsb info to see that it is unowned and a spawn island
3. Register it to a player: /bsb register tastybento
4. Do /bsb info to see that it is owned and a spawn island
5. Do /bsb setspawn and it will say it is already a spawn island.

Expected behavior

If a spawn island is registered to a user, then it should become that player's island and not be the spawn island.

[Poslovitch]

The admin should be told about such behaviour. Might be worth adding a confirmation message.

[wellnesscookie]

Sorry for bumping, but I am afraid this change does not allows me anymore to both set the island as a spawn and keep it registered to a user. Problem is that we use specific range on the spawn island because our spawn is of enormous size and range setting is essential. But since this change we must have a bigger border, yet our skyblock has no island spawn defined anymore.

I believe the island spawn is essential to some of /is expel or similar commands.
This 'bug' had been flagged as 'not a bug' earlier in #990 , but it probably got forgotten.

I have seen no bad outcomes of this feature, as I call it, at least we used it as such.