audit: Function Visibility Can Be Restricted

[Orland0x]

The visibility of the following functions can be restricted in order to save gas:

1. The Space contract's initialize function can be made external.
2. The SpaceManager contract's enableSpace and disableSpace functions could be made external. Alternatively, an internal and external copy of the functions could exist, if inheriting contracts should have access to these functions. Additionally, the isSpaceEnabled function could be made external, and internal uses (e.g. the onlySpace modifier) should directly access the spaces mapping to save gas.