Rename routeQuery to queryString in standard_filter.vue

What does this MR do?

This MR renames the computed properties and methods that use the name routeQuery to querystring instead. Originally, I named it routeQuery because it fetched the querystring value using this.$route.query, but this is the same thing as querystring and having a separate name for it is confusing.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done