Bug 948517, striptags filter blocks should be escaped

While working on #1584, I have noticed that 
```python
{% filter striptags %}{% block page_title_full %}{% block page_title_prefix %}{% endblock %}{% block page_title %}{% endblock %}{% endblock page_title_full %}{% block page_title_suffix %} — Mozilla{% endblock %}{% endfilter %}
{% filter striptags %}{% block page_desc %}{% endblock %}{% endfilter %}
```
strips tags *and replaces `&amp;` with `&`*, while
```
{{ self.page_title_full()|striptags }}
{{ self.page_desc()|striptags }}
```
only strips tags.

In the latter case, ``{{ something }}`` automatically escapes entity references.

In the former case, the string should be escaped with the `e` filter or it leads an invalid HTML syntax.