When an administrator attempts to remove a space's homepage, provide warning.

Problem Definition

Confluence administrators have the ability to remove a space's homepage, which can cause many problems. One of which is that the new placeholder then does not display page tree in the left sidebar and can cause confusion.

Many other things can happen, involving stack trace errors, etc.

Suggested Solution

1. Either we prevent a user from being able to do so,
2. or when the homepage is attempted to be removed, provide a warning about it.