Render social previews for tags with one of the two custom colors set (#16557)

* Initialize compare hex once in controller, fallback if color missing

This PR adds a fallback color choice when there are missing
bg_color_hex or text_color_hex (this is likely going to become more
common as we only show the bg_color_hex in the form).

The choices (black and white) may not be correct - this "custom, or
default" value choice should likely be made in a decorator or on the
model instead.

Addresses an error reported in
https://app.honeybadger.io/projects/66984/faults/83671065

* typo: Attribute name is text_color_hex

app/controllers/social_previews_controller.rb

@@ -38,6 +38,7 @@ class SocialPreviewsController < ApplicationController
 
   def tag
     @tag = Tag.find(params[:id])
+    @compare_hex = Color::CompareHex.new([@tag.bg_color_hex || "#000000", @tag.text_color_hex || "#ffffff"])
 
     set_respond
   end

app/views/social_previews/tag.html.erb

@@ -1,6 +1,6 @@
-<% accent_color = Color::CompareHex.new([@tag.bg_color_hex, @tag.text_color_hex]).biggest %>
-<% color = Color::CompareHex.new([@tag.bg_color_hex, @tag.text_color_hex]).brightness(1.4) %>
-<% dark_color = Color::CompareHex.new([@tag.bg_color_hex, @tag.text_color_hex]).brightness(0.7) %>
+<% accent_color = @compare_hex.biggest %>
+<% color = @compare_hex.brightness(1.4) %>
+<% dark_color = @compare_hex.brightness(0.7) %>
 <% not_so_rand = Random.new(@tag.id) # Using ID as seed ensures we get the same "random" numbers on each page load, Improves caching %>
 <style>
   body {