Better handling of Shortcodes

[SirLouen]

If you use multiple shortcodes on the same page, functions in src/includes/forums/functions.php won't be able to handle correctly all of them and will start triggering errors within each other (and other malfunctioning patterns, like no posts showing for a single forum, etc.)

One very specific example to illustrate this behavior, if we have a [bbp-single-forum] and a [bbp-forum-form] on the same page, bbp_new_forum_handler function $action first condition won't be triggered and since there is no nonce, the error will appear.

We need to see if it's possible to apply some sort of isolation on shortcode output buffer rendering. If this it's impossible, then some control mechanisms should be applied to avoid conflicting shortcodes.

This is not critical right now with shortcodes because they are generally rare to be applied in multiples, given they are not very straightforward, but with the introduction of the future Gutenberg Blocks based on shortcodes, since they are super easy to be applied (drag and drops, this occurrence could be more frequent).