Add server banner to web app, add `GET /api/v2/instance` to REST API (#19294)

Cherry-picked d2528b26b6da34f34b5d7a392e263428d3c09d69

Conflicts:
- `app/serializers/initial_state_serializer.rb`:
  Upstream changed stuff, we had extra attributes.
  Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
  Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
  so updated that file by keeping our extra attributes, and took upstream's
  version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
  Took upstream's version.

1 files changed, 115 insertions, 0 deletions

diff --git a/app/serializers/rest/v1/instance_serializer.rb b/app/serializers/rest/v1/instance_serializer.rb
new file mode 100644
index 000000000..fefbed8ee
--- /dev/null
+++ b/app/serializers/rest/v1/instance_serializer.rb
@@ -0,0 +1,115 @@
+# frozen_string_literal: true
+
+class REST::V1::InstanceSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :uri, :title, :short_description, :description, :email,
+             :version, :urls, :stats, :thumbnail, :max_toot_chars, :poll_limits,
+             :languages, :registrations, :approval_required, :invites_enabled,
+             :configuration
+
+  has_one :contact_account, serializer: REST::AccountSerializer
+
+  has_many :rules, serializer: REST::RuleSerializer
+
+  def uri
+    object.domain
+  end
+
+  def short_description
+    object.description
+  end
+
+  def description
+    Setting.site_description # Legacy
+  end
+
+  def email
+    object.contact.email
+  end
+
+  def contact_account
+    object.contact.account
+  end
+
+  def thumbnail
+    instance_presenter.thumbnail ? full_asset_url(instance_presenter.thumbnail.file.url) : full_pack_url('media/images/preview.png')
+  end
+
+  def max_toot_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
+  def poll_limits
+    {
+      max_options: PollValidator::MAX_OPTIONS,
+      max_option_chars: PollValidator::MAX_OPTION_CHARS,
+      min_expiration: PollValidator::MIN_EXPIRATION,
+      max_expiration: PollValidator::MAX_EXPIRATION,
+    }
+  end
+
+  def stats
+    {
+      user_count: instance_presenter.user_count,
+      status_count: instance_presenter.status_count,
+      domain_count: instance_presenter.domain_count,
+    }
+  end
+
+  def urls
+    { streaming_api: Rails.configuration.x.streaming_api_base_url }
+  end
+
+  def usage
+    {
+      users: {
+        active_month: instance_presenter.active_user_count(4),
+      },
+    }
+  end
+
+  def configuration
+    {
+      statuses: {
+        max_characters: StatusLengthValidator::MAX_CHARS,
+        max_media_attachments: 4,
+        characters_reserved_per_url: StatusLengthValidator::URL_PLACEHOLDER_CHARS,
+      },
+
+      media_attachments: {
+        supported_mime_types: MediaAttachment::IMAGE_MIME_TYPES + MediaAttachment::VIDEO_MIME_TYPES + MediaAttachment::AUDIO_MIME_TYPES,
+        image_size_limit: MediaAttachment::IMAGE_LIMIT,
+        image_matrix_limit: Attachmentable::MAX_MATRIX_LIMIT,
+        video_size_limit: MediaAttachment::VIDEO_LIMIT,
+        video_frame_rate_limit: MediaAttachment::MAX_VIDEO_FRAME_RATE,
+        video_matrix_limit: MediaAttachment::MAX_VIDEO_MATRIX_LIMIT,
+      },
+
+      polls: {
+        max_options: PollValidator::MAX_OPTIONS,
+        max_characters_per_option: PollValidator::MAX_OPTION_CHARS,
+        min_expiration: PollValidator::MIN_EXPIRATION,
+        max_expiration: PollValidator::MAX_EXPIRATION,
+      },
+    }
+  end
+
+  def registrations
+    Setting.registrations_mode != 'none' && !Rails.configuration.x.single_user_mode
+  end
+
+  def approval_required
+    Setting.registrations_mode == 'approved'
+  end
+
+  def invites_enabled
+    UserRole.everyone.can?(:invite_users)
+  end
+
+  private
+
+  def instance_presenter
+    @instance_presenter ||= InstancePresenter.new
+  end
+end