Site isolation headers & redirect fix (#866)

* Fix redirect behaviour

* Generate headers file

* Specific redirect

* Another try

* And again

* Use dedent

package.json

@@ -15,9 +15,11 @@
     "@rollup/plugin-node-resolve": "^9.0.0",
     "@rollup/plugin-replace": "^2.3.4",
     "@surma/rollup-plugin-off-main-thread": "^1.4.2",
+    "@types/dedent": "^0.7.0",
     "@types/node": "^14.14.7",
     "comlink": "^4.3.0",
     "cssnano": "^4.1.10",
+    "dedent": "^0.7.0",
     "del": "^5.1.0",
     "file-drop-element": "^1.0.1",
     "husky": "^4.3.0",