Migrate .html.eex template files to .html.heex

See https://gist.github.com/chrismccord/2ab350f154235ad4a4d0f4de6decba7b#rename-your-htmleex-and-htmlleex-templates-to-htmlheex-optional

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

mix.exs

@@ -100,6 +100,7 @@ defmodule Mobilizon.Mixfile do
       {:phoenix_ecto, "~> 4.0"},
       {:postgrex, ">= 0.15.3"},
       {:phoenix_html, "~> 3.0"},
+      {:phoenix_live_view, "~> 0.16.4"},
       {:gettext, "~> 0.11"},
       {:cowboy, "~> 2.6"},
       {:guardian, "~> 2.0"},