[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/validate/formvalidation.go

@@ -42,6 +42,7 @@ const (
 	maximumSiteTermsLength        = 5000
 	maximumUsernameLength         = 64
 	maximumCustomCSSLength        = 5000
+	maximumEmojiCategoryLength    = 64
 )
 
 // NewPassword returns an error if the given password is not sufficiently strong, or nil if it's ok.
@@ -182,6 +183,14 @@ func EmojiShortcode(shortcode string) error {
 	return nil
 }
 
+// EmojiCategory validates the length of the given category string.
+func EmojiCategory(category string) error {
+	if length := len(category); length > maximumEmojiCategoryLength {
+		return fmt.Errorf("emoji category %s did not pass validation, must be less than %d characters, but provided value was %d characters", category, maximumEmojiCategoryLength, length)
+	}
+	return nil
+}
+
 // SiteTitle ensures that the given site title is within spec.
 func SiteTitle(siteTitle string) error {
 	if length := len([]rune(siteTitle)); length > maximumSiteTitleLength {