FEATURE: Allow themes to define screenshots (#29079)

This commit allows themes to define up to 2 screenshots
in about.json. These should be paths within the theme's
git repository, images with a 1MB max file size and max width 3840x2160.

These screenshots will be downloaded and stored against a theme
field, and we will use these in the redesigned theme grid UI.

These screenshots will be updated when the theme is updated
in the same way the additional theme files are.

For now this is gated behind a hidden `theme_download_screenshots`
site setting, to allow us to test this on a small number of sites without
making other sites make unnecessary uploads.

**Future considerations:**

* We may want to have a specialized naming system for screenshots. E.g. having light.png/dark.png/some_palette.png
* We may want to show more than one screenshot for the theme, maybe in a carousel or reacting to dark mode or color palette changes
* We may want to allow clicking on the theme screenshot to show a lightbox
* We may want to make an optimized thumbnail image for the theme grid

---------

Co-authored-by: Ted Johansson <ted@discourse.org>

config/locales/server.en.yml

@@ -85,6 +85,10 @@ en:
     import_error:
       generic: An error occurred while importing that theme
       upload: "Error creating upload asset: %{name}. %{errors}"
+      screenshot: "Error importing theme screenshots. %{errors}"
+      screenshot_invalid_type: "The theme screenshots must be in one of the following formats: %{accepted_formats}. The screenshot %{file_name} has an invalid format."
+      screenshot_invalid_size: "The theme screenshots must be less than %{max_size}. The screenshot %{file_name} is too large."
+      screenshot_invalid_dimensions: "The theme screenshots must be maximum %{max_width}x%{max_height}. The screenshot %{file_name} exceeds this. Its dimensions are %{width}x%{height}."
       about_json: "Import Error: about.json does not exist, or is invalid. Are you sure this is a Discourse Theme?"
       about_json_too_big: "Import Error: about.json is bigger than the %{limit} limit."
       about_json_values: "about.json contains invalid values: %{errors}"

config/site_settings.yml

@@ -1525,6 +1525,9 @@ files:
     default: "wasm|jpg|jpeg|png|woff|woff2|svg|eot|ttf|otf|gif|webp|avif|js"
     type: list
     list_type: file_types
+  theme_download_screenshots:
+    default: false
+    hidden: true
   authorized_extensions:
     client: true
     default: "jpg|jpeg|png|gif|heic|heif|webp|avif"