Complete the Api

Currently the GraphQL Api in decidim lacks of many desired features. 1. Lots of fields (publicly available in the web) are not available if asked via API. For example, description field for a participatory process. 2. Components needs more specific-type definitions (i.e: you cannot extract pages component content from a participatory space). 3. Individual participatory spaces and components should be searchable via slug as well (when applicable). 4. Listing participatory processes and attributes should be searchable via many attributes: find by scope, find by hashtag, etc. 5. Listing elements should be ordered in a consistent way. Ideally this order should be specified by the query. I intent (as Platoniq organization) to continue improve the API. I'll try to stick the the contribution rules and, of course, let me know anything that you want me to improve in order to accept this and subsequents pull-requests.