View configuration UI: Add indicator denoting rewritten fields

As a new, separate issue to #5261 the request is similar to that issue where an indicator is added to show whether a field has been rewritten. This will aid in helping a site builder understand why a field in the output, isn't acting the way they expect. Having an indicator that a field is rewritten, gives a visual representation of what the field is actually outputting.

Things to do: - Add a class "views-field-rewritten" to the field link if it is rewritten, similar to how an excluded field has the class "views-field-excluded" to allow custom styling on the link based on the fact that the link is rewritten. - Add a span to house any icons that may be used as a visual indicator

Possible ideas for icons:

[file-contract] [cog] [wrench] [exclamation] [pen-fancy] [paint-brush]

IMO the file-contract makes the most sense out of these, but I'm not sold on any one of them.

If I missed anything, feel free to add it.