Remove deprecated attributes in html templates

[msn0]

In ./src/selena/boards/templates/boards/*.html scripts are included using

<script language="javascript" type="text/javascript" src="..."></script>

The language attribute on the script element is deprecated and produces warning in W3C Validator. We can safely omit it. Moreover, templates extends base.html which is HTML5 document thus type="text/javascript" is completely unnecessary.

[msn0]

If you are interested in this change its already done in https://github.com/allegro/selena/pull/30