Remove alt attribute in avatar images

These images are always displayed next to a username, meaning people
using screen readers were hearing the same username twice in a row.

Even though we're about to replace the initialjs gem, we're making this
change in case so we've got one more test and we can check everything
keeps working after replacing the gem.

app/components/shared/avatar_component.rb

@@ -10,7 +10,7 @@ class Shared::AvatarComponent < ApplicationComponent
   private
 
     def default_options
-      { background_color: colors[seed % colors.size] }
+      { background_color: colors[seed % colors.size], alt: "" }
     end
 
     def options

spec/components/shared/avatar_component_spec.rb

@@ -0,0 +1,10 @@
+require "rails_helper"
+
+describe Shared::AvatarComponent do
+  it "does not contain redundant text already present around it" do
+    render_inline Shared::AvatarComponent.new(double(id: 1, name: "Johnny"))
+
+    expect(page).to have_css "img", count: 1
+    expect(page).to have_css "img[alt='']"
+  end
+end