core&ui: add settings to replace Gravatar URL (#279)

implement #282

packages/hydrooj/src/lib/avatar.ts

@@ -3,7 +3,8 @@ import { md5 } from './crypto';
 type AvatarProvider = (src: string, size: number) => string;
 
 export const providers: Record<string, AvatarProvider> = {
-    gravatar: (email, size) => `//cn.gravatar.com/avatar/${md5((email || '').toString().trim().toLowerCase())}?d=mm&s=${size || 32}`,
+    gravatar: (email, size) => `${global.Hydro.model.system.get('avatar.gravatar_url')}`
+    + `${md5((email || '').toString().trim().toLowerCase())}?d=mm&s=${size || 32}`,
     qq: (id) => `//q1.qlogo.cn/g?b=qq&nk=${(/(\d+)/g.exec(id) || ['', ''])[1]}&s=160`,
     github: (id, size) => `//github.com/${id}.png?size=${Math.min(size, 460)}`,
     url: (url) => url,