Fix angular dependency injection scheme

@philip.abbet could you try this on staging?

beat/web/reports/static/reports/app/services/plotService.js

@@ -117,7 +117,9 @@ angular.module('reportApp').factory('PlotService', ['UrlService', function(UrlSe
     };
 
     const fetchDownload = (requestData, contentType) => {
-        const urlPrefix = '';
+        let prefix = new URL(UrlService.getPrefix()).pathname;
+        if(prefix.endsWith('/'))
+            prefix = prefix.slice(0, -1);
 
         // override the 'merged' property to always request the merged version.
         // requested the unmerged version only downloads the first image.
@@ -126,7 +128,7 @@ angular.module('reportApp').factory('PlotService', ['UrlService', function(UrlSe
         return new Promise((resolve, reject) => {
             beat.experiments.utils.getPlotData(
                 // url_prefix
-                urlPrefix,
+                prefix,
                 // spread out the request data to fill the next 3 spots
                 ...requestData,
                 // content type: png, jpeg, pdf
@@ -142,14 +144,14 @@ angular.module('reportApp').factory('PlotService', ['UrlService', function(UrlSe
 
     // makes the call to the server, via some helper funcs found in the global namespace
     const fetchRender = (requestData, containerId, onRenderCallback) => {
-        // the 'url_prefix' field found throughout the BEAT code is confusing,
-        // but it seems to always be an empty string now
-        const urlPrefix = '';//UrlService.getApiSegment().split('/').filter(s => s.length > 0).join('/');
+        let prefix = new URL(UrlService.getPrefix()).pathname;
+        if(prefix.endsWith('/'))
+            prefix = prefix.slice(0, -1);
 
         // this func *cannot* be promisified! the callback is fired whenever the plot is re-rendered.
         beat.experiments.utils.displayPlot(
             // url_prefix
-            urlPrefix,
+            prefix,
             // element to append render to
 	    document.getElementById(`${ containerId }`),
             // spread out the request data to fill the next 3 spots