Use display name in new user email

[ctml91]

Bug Report

Current Behavior
New user emails shows username

Steps to Reproduce

1. Use a display name driver (nickname)
2. Subscribe to new user emails as an admin
3. Check an email when a new user registers
4. The email contains the username, it would be preferable to have this use the display name instead

Expected Behavior
The email should display the users display name

Environment

• Flarum version: beta15

[w3Abhishek]

@FriendsOfFlarum can i work on this issue?

[imorland]

@w3Abhishek abolutely! Just submit a PR once you're happy with the changes and we'll take a look and if everything is good we'll merge and release!