Set up user-agent for PCS stored requests

Endpoints affected:
* summary
* mobile-html
* media-list

Top level config is not maintained when custom headers are
defined on hyper requests. Also top level config is not available
on template expansion, thus we need to redefine the user-agent in
multiple places.

v1/pcs/stored_endpoint.js

@@ -8,6 +8,7 @@ class PCSEndpoint {
     constructor(options) {
         this._options = options;
         this._disabled_storage = options.disabled_storage || false;
+        this.user_agent = 'RESTBase/WMF';
     }
 
     _injectCacheControl(res) {
@@ -110,7 +111,8 @@ class PCSEndpoint {
         return hyper.get({
             uri: new URI(serviceURI),
             headers: {
-                'accept-language': req.headers['accept-language']
+                'accept-language': req.headers['accept-language'],
+                'user-agent': this.user_agent
             }
         }).tap(() => hyper.metrics.endTiming([
             'pcs_fetch_latency',

v1/summary_new.yaml

@@ -130,6 +130,7 @@ paths:
               method: get
               headers:
                 accept-language: '{{accept-language}}'
+                user-agent: 'RESTBase/WMF'
               uri: '{{options.host}}/{domain}/v1/page/summary/{title}'
             response:
               status: '{{extract.status}}'