Color safeSum in forest green as used in the Grayson Dot (#535)

## Summary
The chart includes the Grayson Dot - a different color green applied to
green dots with more than 7d buffer. The iOS app colors the `safeSum`
using colors based on the colors of the dots. This text appears in both
the gallery in a goal's cell as well as in the goal screen itself.
Previously the same green was being used for all `safeSum` with a
`safeBuf` greater than three.

Implements #206

## Validation
Ran app in the simulator using the new colors and logic. Made
screenshots.

Author: krugerk

BeeKit/GoalExtensions.swift

@@ -55,17 +55,18 @@ extension Goal {
     }
 
     public var countdownColor :UIColor {
-        let buf = self.safeBuf
-        if buf < 1 {
-            return UIColor.Beeminder.red
+        switch self.safeBuf {
+        case ..<1:
+            return UIColor.Beeminder.SafetyBuffer.red
+        case ..<2:
+            return UIColor.Beeminder.SafetyBuffer.orange
+        case ..<3:
+            return UIColor.Beeminder.SafetyBuffer.blue
+        case ..<7:
+            return UIColor.Beeminder.SafetyBuffer.green
+        default:
+            return UIColor.Beeminder.SafetyBuffer.forestGreen
         }
-        else if buf < 2 {
-            return UIColor.Beeminder.orange
-        }
-        else if buf < 3 {
-            return UIColor.Beeminder.blue
-        }
-        return UIColor.Beeminder.green
     }
 
     public var hideDataEntry: Bool {

BeeKit/UI/UIColorExtension.swift

@@ -12,20 +12,21 @@ import UIKit
 extension UIColor {
 
     public struct Beeminder {
-        public static let green = UIColor(red: 81.0/255.0,
-                                          green: 163.0/255.0,
-                                          blue: 81.0/255.0,
-                                          alpha: 1)
-        
-        public static let blue: UIColor = .systemBlue
-        public static let orange: UIColor = .systemOrange
         public static let red: UIColor = .systemRed
 
-        public static let gray = UIColor(white: 0.7, alpha: 1.0)
+        public static let gray = UIColor.systemGray
 
         public static let yellow = UIColor(red: 255.0/255.0,
                                            green: 217.0/255.0,
                                            blue: 17.0/255.0,
                                            alpha: 1)
+        
+        public struct SafetyBuffer {
+            public static let red: UIColor = .systemRed // .init(red: 1, green: 0, blue: 0, alpha: 1)
+            public static let orange: UIColor = .systemOrange // .init(red: 1, green: 165/255.0, blue: 00, alpha: 1)
+            public static let blue: UIColor = .systemBlue // .init(red: 63/255.0, green: 63/255.0, blue: 1, alpha: 1)
+            public static let green: UIColor = .systemGreen // .init(red: 0, green: 170/255.0, blue: 0, alpha: 1)
+            public static let forestGreen: UIColor = .init(red: 34/255.0, green: 139/255.0, blue: 34/255.0, alpha: 1)
+        }
     }
 }