Add reset search buttons to search forms

**Is your feature request related to a problem? Please describe.**

Decidim implements multiple search forms. There is no easy way to reset the search criteria back to defaults if you don't know how to browse to the same page again.

**Describe the solution you'd like**

I suggest adding a "Reset search" button to all search forms.

**Describe alternatives you've considered**

Browsing back to the same page using navigation but this is not very intuitive given the extremely complex navigational structure of Decidim.

**Additional context**

This is an issue that would also improve the accessibility of the system. I understand it is not required to pass accessibility reviews but it would improve accessibility. **Does this issue could impact on users private data?**

No. **Funded by**

N/A