Add missing underscores to bb search cache_keys

[nickchomey]

Jira Issue:

https://support.buddyboss.com/support/tickets/182781

In a few places within the BB Search Component, there is a missing _ in the cache_key. This results in some cache_keys looking like this:

"bb_search_term_total_match_count_searchtermtopic" "bb_search_term_total_match_count_searchtermforum" "bb_search_term_total_match_count_searchtermreply" etc...

Rather than: "bb_search_term_total_match_count_searchterm_topic" "bb_search_term_total_match_count_searchterm_forum" "bb_search_term_total_match_count_searchterm_reply"

It might not make any difference to the search performance/results, but it makes it difficult to read/browse the cache while debugging things

The same thing appears to happen in a couple other spots within the BB Network Search component. I've added a bit of code to add underscores where necessary. However, there may very well be other parts of the broader BB codebase where this problem still occurs.