Add structure in API for meta tags to replace others

There have been two issues recently (#2267501: Replace twitter:image with twitter:image:src, #2121437: Change from "Copyright" to "Rights" per w3c/whatwg metatag guidelines) where a meta tag was renamed. While it's simple to do an update script to update values in the {metatag} table, that does not resolve the problem that exported configurations will loose their existing values, i.e. the configurations for the old old meta tags will disappear. I suggest adding something like a 'replaces' value to the meta tag definition, which would ben an array containing the names of meta tags that were replaced by the current tag, so if settings for the old values were found they would be used as values for the new meta tag instead.