[NEW] Forums: Add possibility to create sub-forums

Description

The forum features are slightly underdeveloped however. Any robust forum software must have the flexibility to be able to create sub-forums (i.e. a forum within a forum) if needed.

Without this functionality forum administrators using Tiki for their forum are forced to use workarounds such as having to add another forum called "Other" that is meant to be a catch-all for anything that is not defined by the existing forums.

For more information about Sub Forum: https://tiki.org/forumthread32313-Subforum