Refactor: BitchatMessage (#610)

* Extract BitchatMessage into a separate file

* Convert `fromBinaryPayload` to `convenience init?`

* Extract message dedup into an extension

* Remove dead `formatMessageContent`

* Minor refactor of timestamp and username formatting

* Remove dead `getSenderColor`

bitchatTests/BLEServiceTests.swift

@@ -215,7 +215,7 @@ final class BLEServiceTests: XCTestCase {
         let expectation = XCTestExpectation(description: "Delivery handler called")
         
         service.packetDeliveryHandler = { packet in
-            if let msg = BitchatMessage.fromBinaryPayload(packet.payload) {
+            if let msg = BitchatMessage(packet.payload) {
                 XCTAssertEqual(msg.content, "Test delivery")
                 expectation.fulfill()
             }