Centralize GIF parsing in a helper like for reactions

lib/helpers/gif_helper.dart

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+class GifHelper {
+  /// Parse a known GIF format, which can be any of:
+  /// g:GIFID
+  /// https://media.giphy.com/media/GIFID/giphy.gif
+  /// https://giphy.com/gifs/Optional-title-with-dashes-GIFID
+  ///
+  /// GIFID is a Giphy GIF ID. The https:// is optional (and
+  /// can also be http://). The giphy.com/gifs form can also
+  /// include a trailing slash.
+  ///
+  /// Returns null if text is not a valid GIF format
+  static String? parseGifId(String text) {
+    final trimmed = text.trim();
+    final match = RegExp(r'^g:([A-Za-z0-9_-]+)$').firstMatch(trimmed);
+    if (match != null) {
+      return match.group(1);
+    }
+    final directUrlMatch = RegExp(
+      r'^(?:https?:\/\/)?media\.giphy\.com\/media\/([A-Za-z0-9_-]+)\/giphy\.gif$',
+    ).firstMatch(trimmed);
+    if (directUrlMatch != null) {
+      return directUrlMatch.group(1);
+    }
+    // Giphy understands page URLs with just the ID, or any string and a
+    // dash before the ID, and redirects to a page with a dash-separated
+    // title, a dash, and the ID. IDs in this form *probably* can't
+    // contain dashes.
+    final pageMatch = RegExp(
+      r'^(?:https?:\/\/)?giphy\.com\/gifs\/(?:[^/?]*-)?([A-Za-z0-9_]+)\/?$',
+    ).firstMatch(trimmed);
+    return pageMatch?.group(1);
+  }
+}