[feature] Allow newly uploaded emojis to be placed in categories (#939)

* [feature] Add emoji categories GET
Serialize emojis in appropriate categories; make it possible to get categories via the admin API

* [feature] Create (or use existing) category for new emoji uploads

* fix lint issue

* update misleading line in swagger docs

internal/typeutils/internaltofrontend.go

@@ -356,12 +356,24 @@ func (c *converter) MentionToAPIMention(ctx context.Context, m *gtsmodel.Mention
 }
 
 func (c *converter) EmojiToAPIEmoji(ctx context.Context, e *gtsmodel.Emoji) (model.Emoji, error) {
+	var category string
+	if e.CategoryID != "" {
+		if e.Category == nil {
+			var err error
+			e.Category, err = c.db.GetEmojiCategory(ctx, e.CategoryID)
+			if err != nil {
+				return model.Emoji{}, err
+			}
+		}
+		category = e.Category.Name
+	}
+
 	return model.Emoji{
 		Shortcode:       e.Shortcode,
 		URL:             e.ImageURL,
 		StaticURL:       e.ImageStaticURL,
 		VisibleInPicker: *e.VisibleInPicker,
-		Category:        e.CategoryID,
+		Category:        category,
 	}, nil
 }
 
@@ -383,6 +395,13 @@ func (c *converter) EmojiToAdminAPIEmoji(ctx context.Context, e *gtsmodel.Emoji)
 	}, nil
 }
 
+func (c *converter) EmojiCategoryToAPIEmojiCategory(ctx context.Context, category *gtsmodel.EmojiCategory) (*model.EmojiCategory, error) {
+	return &model.EmojiCategory{
+		ID:   category.ID,
+		Name: category.Name,
+	}, nil
+}
+
 func (c *converter) TagToAPITag(ctx context.Context, t *gtsmodel.Tag) (model.Tag, error) {
 	return model.Tag{
 		Name: t.Name,