Add a stats category for static badges (#3168)

Right now they're showing up in "other," though I expect they make up
most of that category.

https://github.com/badges/shields/issues/966#issuecomment-469851361

core/base-service/categories.js

@@ -6,7 +6,7 @@ const categories = require('../../services/categories')
 const isRealCategory = Joi.equal(categories.map(({ id }) => id)).required()
 
 const isValidCategory = Joi.alternatives()
-  .try(isRealCategory, Joi.equal('debug', 'dynamic').required())
+  .try(isRealCategory, Joi.equal('debug', 'dynamic', 'static').required())
   .required()
 
 function assertValidCategory(category, message = undefined) {

services/static-badge/query-string-static.service.js

@@ -9,7 +9,7 @@ const queryParamSchema = Joi.object({
 
 module.exports = class QueryStringStaticBadge extends BaseStaticService {
   static get category() {
-    return 'other'
+    return 'static'
   }
 
   static get route() {

services/static-badge/static-badge.service.js

@@ -5,7 +5,7 @@ const { escapeFormat } = require('../../core/badge-urls/path-helpers')
 
 module.exports = class StaticBadge extends BaseStaticService {
   static get category() {
-    return 'other'
+    return 'static'
   }
 
   static get route() {