deps: upgrade phoenix_html and add phoenix_html_helpers

[meagharty]

Upgrades to phoenix_html to v4+ which moves HTML helpers to phoenix_html_helpers.

From phoenix_html changelog:

v4.0.0 (2023-12-19)

This version removes deprecated functionality and moved all HTML helpers to a separate library. HTML Helpers are no longer used in new apps from Phoenix v1.7. Older applications who wish to maintain compatibility, add {:phoenix_html_helpers, "~> 1.0"} to your mix.exs and then replace use Phoenix.HTML in your applications by:

import Phoenix.HTML
import Phoenix.HTML.Form
use PhoenixHTMLHelpers

[meagharty]

@fidr do you have thoughts on this?

[fidr]

Looks good, thanks for taking the time to do this!