Refine and/or query validation for different searches

Created by: rvantonder

One piece of validation that is preserved in and/or queries is detecting whether a field exists. For example, blahblah:value will raise an alert that blahblah does not exist. This is fine for regex search mode, but in structural search, the pattern blahblah:[hole] is quite common, and here we'll raise an alert. This effectively means that all structural search queries need to be quoted, which is annoying. I want to remove this validation for structural search, but not regex mode (unless there are good reasons to do so for regex too). It'll make it a lot more usable.

Currently, the search-specific validation (e.g., ensure that case cannot be set with structural search) has not been migrated yet.