Get thumbnail from app (#68)

* Renamed multipart filed name 'files' to 'assetData'. 
* Added an additional field name of 'thumbnailData' to multipart form.
* Implemented upload mechanism for thumbnail directly from the mobile client.
* Removed dead code
* Implemented a version checking mechanism.

mobile/lib/shared/models/server_version.model.dart

@@ -0,0 +1,72 @@
+import 'dart:convert';
+
+class ServerVersion {
+  final int major;
+  final int minor;
+  final int patch;
+  final int build;
+
+  ServerVersion({
+    required this.major,
+    required this.minor,
+    required this.patch,
+    required this.build,
+  });
+
+  ServerVersion copyWith({
+    int? major,
+    int? minor,
+    int? patch,
+    int? build,
+  }) {
+    return ServerVersion(
+      major: major ?? this.major,
+      minor: minor ?? this.minor,
+      patch: patch ?? this.patch,
+      build: build ?? this.build,
+    );
+  }
+
+  Map<String, dynamic> toMap() {
+    return {
+      'major': major,
+      'minor': minor,
+      'patch': patch,
+      'build': build,
+    };
+  }
+
+  factory ServerVersion.fromMap(Map<String, dynamic> map) {
+    return ServerVersion(
+      major: map['major']?.toInt() ?? 0,
+      minor: map['minor']?.toInt() ?? 0,
+      patch: map['patch']?.toInt() ?? 0,
+      build: map['build']?.toInt() ?? 0,
+    );
+  }
+
+  String toJson() => json.encode(toMap());
+
+  factory ServerVersion.fromJson(String source) => ServerVersion.fromMap(json.decode(source));
+
+  @override
+  String toString() {
+    return 'ServerVersion(major: $major, minor: $minor, patch: $patch, build: $build)';
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+
+    return other is ServerVersion &&
+        other.major == major &&
+        other.minor == minor &&
+        other.patch == patch &&
+        other.build == build;
+  }
+
+  @override
+  int get hashCode {
+    return major.hashCode ^ minor.hashCode ^ patch.hashCode ^ build.hashCode;
+  }
+}