Put reaction and GIF helpers in charge of encoding

lib/helpers/gif_helper.dart

@@ -9,7 +9,7 @@ class GifHelper {
   /// include a trailing slash.
   ///
   /// Returns null if text is not a valid GIF format
-  static String? parseGifId(String text) {
+  static String? parseGif(String text) {
     final trimmed = text.trim();
     final match = RegExp(r'^g:([A-Za-z0-9_-]+)$').firstMatch(trimmed);
     if (match != null) {
@@ -30,4 +30,9 @@ class GifHelper {
     ).firstMatch(trimmed);
     return pageMatch?.group(1);
   }
+  
+  /// Encode a GIF in a format that parseGif() can parse.
+  static String encodeGif(String gifId) {
+    return 'g:$gifId';
+  }
 }

lib/helpers/reaction_helper.dart

@@ -109,4 +109,9 @@ class ReactionHelper {
 
     return ReactionInfo(targetHash: match.group(1)!, emoji: emoji);
   }
+  
+  /// Encode a reaction message that parseReaction() can parse.
+  static String encodeReaction(String hash, String emojiIndex) {
+    return 'r:$hash:$emojiIndex';
+  }
 }