Fixed linter errors and warnings.

browser-plugins/webextension/add_uri.html

@@ -7,7 +7,7 @@
         <label for="tags">Tags, comma separated:</label>
         <input type="text" id="tags" autofocus>
         <input type="button" id="button" value="Add URI">
-        <div id="error"></div>
+        <strong id="error"></strong>
 
         <script src="launch.js"></script>
     </body>

browser-plugins/webextension/launch.js

@@ -25,8 +25,7 @@ function onResponse(response) {
     }
     else
     {
-        document.getElementById("error").innerHTML =
-            "<strong>" + response +  "</strong>";
+        document.getElementById("error").textContent = response;
     }
 
 }

browser-plugins/webextension/manifest.json

@@ -16,7 +16,7 @@
         "gecko":
         {
             "id": "remwharead@tastytea.de",
-            "strict_min_version": "50.0"
+            "strict_min_version": "57.0"
         }
     },