[FIX] qweb: remove comments in xml strings before parsing

It was done in loadTemplates, but this does not work if we want to add
templates from another source.

tests/qweb/__snapshots__/qweb.test.ts.snap

@@ -768,6 +768,18 @@ exports[`static templates empty div 1`] = `
 }"
 `;
 
+exports[`static templates ignore comments 1`] = `
+"function anonymous(context,extra
+) {
+    var h = this.h;
+    let c1 = [], p1 = {key:1};
+    var vn1 = h('div', p1, c1);
+    result = vn1;
+    c1.push({text: \`hello owl\`});
+    return vn1;
+}"
+`;
+
 exports[`static templates simple dynamic value 1`] = `
 "function anonymous(context,extra
 ) {

tests/qweb/qweb.test.ts

@@ -49,6 +49,12 @@ describe("static templates", () => {
     qweb.addTemplate("test", "<div><span>word</span></div>");
     expect(renderToString(qweb, "test")).toBe("<div><span>word</span></div>");
   });
+
+  test("ignore comments", () => {
+    qweb.addTemplate("test", "<div>hello <!-- comment-->owl</div>");
+    expect(renderToString(qweb, "test")).toBe("<div>hello owl</div>");
+  });
+
 });
 
 describe("error handling", () => {