Use 'type' (enum parameter) instead of 'standalone' (bool parameter)

[koromodako]

Replace standalone with type being an enumeration of three values:

• static: a static challenge can be solved offline once resources have been downloaded from the challenge platform.
• simple: a simple challenge requires a server but the intended exploit does not result in server-side command/code execution.
• sandboxed: a sandboxed challenge requires a server and the exploit results in partial/full compromission of the server (RCE/ACE/...).

[koromodako]

Each 'type' will have a configurable directory layout defined within :

• the global configuration (default base of repo config)
• and in the repo configuration file (layout to be used within this repo)

[koromodako]

mkCTF 6.0.0 implements this feature.