In commit f374478dd, we enabled the possibility to use HTML in the
search results translations in order to add a <strong> tag to these
results. However, that meant we were also allowing HTML tags inside the
search term itself, and so it was possible to inject HTML on the page.
Stripping the HTML tags solves the issue.
Note the issue wasn't a high severity issue because tags such as
`<script>` weren't allowed since we were using the `sanitize` helper.
We were using very similar code for proposals, debates and investments,
so we might as well share the code between them.
Note we're using the `proposals.index.search_results` key even for
debates and investments. This will still work because the translations
shared the same text, but IMHO we should rename the key to something
like `shared.search_results_summary`. We aren't doing so because we'd
lose all the existing translations.
