Add forum search (and improve search keys as array handling)

[johnjamesjacoby]

Topic & Reply searching is currently supported, but sites with many forums do not have a specific way to search for existing forums.

Let's add support for this.

While we're at it, let's also better handle query arguments that are in the shape of an array rather than a string, as there have been a few notices hitting the Dotorg logs.

[johnjamesjacoby]

Searching a forum listing for forums with "gen" in them

[johnjamesjacoby]

In r6903:

Search: add support for fs query-arg to search forums list:

• Introduces bbp_sanitize_search_request() to encapsulate duplicate code across forums/topics/replies
• Introduces bbp_get_search_type_ids() to stub out future enhancements (tags, users, etc...)
• Use these new functions where intended
• Update bbp_get_search_terms() to loop through known search-type IDs

This commit also fixes debug notices that would happen when these query arguments were not explicitly strings.

Fixes #3254.