Decouple bbp_admin_reset_handler handler

[espellcaste]

Right now, bbp_admin_reset_handler() cannot be used on CLI contexts because of its checks.

I didn't want to duplicate its functionality on ​https://github.com/bbpress/wp-cli-bbpress, so I decoupled this function to allow its usage there.

After this change is merged, I can call bbp_admin_reset_bbpress() on CLI.

[espellcaste]

Relate to #3058.

[johnjamesjacoby]

In 6750:

Admin: split reset handler into 2 functions (handler and logic)

This change improves compatibility with future WP CLI initiatives, specifically allowing removing all bbPress data.

Fixes #3173. Props espellcaste.