Better handling of filter expiration date (#212)

* Better handling of filter expiration date
* Simplify Thread and Home Timeline filtering

mastodon/src/main/java/org/joinmastodon/android/model/Filter.java

@@ -23,7 +23,7 @@ public class Filter extends BaseModel{
 	public Instant expiresAt;
 	public boolean irreversible;
 	public boolean wholeWord;
-	public String filterAction;
+	public FilterAction filterAction;
 
 	@SerializedName("context")
 	private List<FilterContext> _context;
@@ -79,4 +79,11 @@ public class Filter extends BaseModel{
 		@SerializedName("thread")
 		THREAD
 	}
+
+	public enum FilterAction{
+		@SerializedName("hide")
+		HIDE,
+		@SerializedName("warn")
+		WARN
+	}
 }