Fix XSS security issue in tag_selector form

How to reproduce:

1- Log in.
2- Go to some view with the tag selector form, like "Home".
3- Write some JavaScript in one of the form fields. Something like: <script>alert("Hi! This is a XSS attack")</script>
4- Click "Add" button.
5- Enjoy.

Tested in version 0.7.43