Create aliases and an implication for tag: straddle_split (and an implication for tag: split)

BUR #22394 has been approved by @nonamethanks.

create alias middle_split -> straddle_split
create implication straddle_split -> split

Straddle splits is also often called Middle splits.

As for the implication, straddle split (straddle splits) is also a variant of splits (split), which itself requires flexibility.

Removed split -> flexible because there's several posts under split failure like post #4508306 where it wouldn't apply.

The bulk update request #22394 (forum #264974) has been approved by @nonamethanks.