Xcode has undesirable default header comments, and now you can customize them.
This adds an `IDETemplateMacros.plist` file to the `xcworkspace` so that new files created
in the workspace have the same, consistent default header comments automatically added:
```
//
// Copyright (c) 2018 Open Whisper Systems. All rights reserved.
//
```
This will prevent contributors from either (1) committing Xcode's default, or
(2) having to manually copy-pasta the standard comment headers for every new file added.
[See here for more details](https://oleb.net/blog/2017/07/xcode-9-text-macros/)
Niels Andriesse
Jesse Squires