Add dropbox CSP directive if configured and make button clickable

The lack of a 'preventDefault' on the click event handler resulted in the dropbox link being unclickable.
Furthermore because of a missing CSP rule, the dropbox script couldn't be loaded. The dropbox origin is now added to the CSP script sources if dropbox integration is configured.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

public/js/index.js

@@ -996,7 +996,8 @@ ui.toolbar.export.snippet.click(function () {
     })
 })
 // import from dropbox
-ui.toolbar.import.dropbox.click(function () {
+ui.toolbar.import.dropbox.click(function (event) {
+  event.preventDefault()
   var options = {
     success: function (files) {
       ui.spinner.show()