Improve UX when listing suggestions

[ggiraldez]

Closes #695

• Show a message in the sidebar while creating a new provider
• Refactor: simplify and improve suggestions handling code in the client
• Render marker icons according to the type of suggestion, both in the map and the suggestions list
• When selecting suggestions, render the coverage of the suggestion
• Dim existing providers and changes while selecting from a list of suggestions
• Unselect providers when selecting suggestions and viceversa
• Restore UI state after creating a new change from a suggestion or from clicking on the map
• Hide delete button in change dialog if the change is new