fix checkboxes shown as checked when they're not with mysql

The issue occurs because boolean/tinyint values are retrieved from mysql
as strings, and in php/js all non-empty strings are cast as boolean
true.

Current PDO mysql driver doesn't support `PDO::ATTR_STRINGIFY_FETCHES =
false`, and if I disable prepare-emulation so it uses the native MySQL
driver instead which supposedly does support it, prepare statements no
longer play nice with named parameters.

Every remaining clean solution that comes to mind that can cover all
cases, just for MySQL, adds an annoying amount of additional code /
overhead.

As long as the `App.FormFields.checkbox_tag()` JS function is the only
one suffering from the lack of conversion, I'll go with easy ugly over
here.

js/App.js

@@ -43,8 +43,9 @@ const App = {
          return this.button_tag(value, "", {...{onclick: "App.dialogOf(this).hide()"}, ...attributes});
       },
       checkbox_tag: function(name, checked = false, value = "", attributes = {}, id = "") {
+         // checked !== '0' prevents mysql "boolean" false to be implicitly cast as true
          return `<input dojoType="dijit.form.CheckBox" type="checkbox" name="${App.escapeHtml(name)}"
-                     ${checked ? "checked" : ""}
+                     ${checked !== '0' && checked ? "checked" : ""}
                      ${value ? `value="${App.escapeHtml(value)}"` : ""}
                      ${this.attributes_to_string(attributes)} id="${App.escapeHtml(id)}">`
       },