mix phx.gen.auth: add "Delete Account" button to generated settings page

[nbw]

The Settings page generated with mix phx.gen auth creates a form to update a user's email and another to set a new password. I've been using the generator with a few new apps and I noticed that I had to implement delete account in each case. It feels like this should come out of the box with the generator. Refer to the red "Delete Account" button in the screenshot below.

👋 Note: I've implemented it for the liveview auth flow. I'd be willing to add it to the static template/controller based flow too if the response to this PR is positive.

Screenshots

Demo

demo.mp4

Local testing instructions

1. Pull phoenix project somewhere locally
2. create a new phoenix project
3. update the phoenix dependency (mix.exs) to point to the local copy of phoenix:

# relative path to the local copy of phoenix
{:phoenix, path: "../phoenix", override: true},

4. Run mix phx.gen.auth Accounts User users