Pluggable ACP

[Nicolas]

Proposition:

ACP as a plugin/module. Introduce new namespace, for example nodebb-acp-somename

So you will have:

• For themes: nodebb-theme-somename
• For plugins: nodebb-plugin-somename
• For ACP: nodebb-acp-somename

Encapsulate current ACP to acp-module, and see, in near future, how 3rd party ACP panel will popup (Angular based, or Material based, or something another...)