Replace Filter class with expression structure

This is in order to be able to support UPPER() and LOWER() functions in
queries which are needed for case-insensitive search.

client/components/summon-button.js

@@ -4,12 +4,19 @@ import m from "mithril";
 import * as taskQueue from "../task-queue";
 import * as store from "../store";
 import * as notifications from "../notifications";
-import * as filterParser from "../../common/filter-parser";
+import * as funcCache from "../../common/func-cache";
+import * as expression from "../../common/expression";
+
+const parseParameter = funcCache.getter(p => {
+  p = expression.parse(p);
+  if (Array.isArray(p) && p[0] === "PARAM") p = p[1];
+  return p;
+});
 
 const component = {
   oninit: vnode => {
     vnode.state.parameters = Object.values(vnode.attrs.parameters).map(
-      parameter => filterParser.parseParameter(parameter)
+      parameter => parseParameter(parameter)
     );
   },
   view: vnode => {