[milo] remove linkBase and calculate links directly.

milo/buildsource/swarming/expectations/build-exception.json

 {
   "Summary": {
     "Label": "2d8d62f6be545b10",
     "Status": "Failure",
     "Source": {
       "Label": "Task 2d8d62f6be545b10",
       "URL": "https://example.com/task?id=2d8d62f6be545b10\u0026show_raw=1\u0026wide_logs=true"
     },
     "MainLink": [
       {
         "Label": "Task 2d8d62f6be545b10",
         "URL": "https://example.com/task?id=2d8d62f6be545b10\u0026show_raw=1\u0026wide_logs=true"
       }
     ],
     "SubLink": [
       [
         {
           "Label": "stdout",
-          "URL": "foo/stdout"
+          "URL": "/swarming/task/build-exception/stdout"
         }
       ]
     ],
     "Started": "2016-03-14T10:00:01.85071Z",
     "Finished": "2016-03-14T11:00:18.48602Z",
     "Duration": 3616635310000,
     "Type": "Recipe",
     "LevelsDeep": 0,
     "Verbosity": 2,
     "Text": [
       "InfraFailure bot_update",
       "InfraFailure Uncaught Exception"
     ]
   },
   "SourceStamp": null,
   "Components": [
     {
       "Label": "recipe bootstrap",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/recipe_bootstrap/0/stdout"
+          "URL": "/swarming/task/build-exception/steps/recipe_bootstrap/0/stdout"
         }
       ],
       "Started": "2016-03-14T10:00:02Z",
       "Finished": "2016-03-14T10:00:09Z",
       "Duration": 7000000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 1,
       "Text": null
     },
     {
       "Label": "setup_build",
       "Status": "Success",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/setup_build/0/stdout"
+          "URL": "/swarming/task/build-exception/steps/setup_build/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "run_recipe",
-            "URL": "foo/steps/setup_build/0/logs/run_recipe/0"
+            "URL": "/swarming/task/build-exception/steps/setup_build/0/logs/run_recipe/0"
           }
         ]
       ],
       "Started": "2016-03-14T10:00:03Z",
       "Finished": "2016-03-14T10:00:04Z",
       "Duration": 1000000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 1,
       "Text": [
         "running recipe: \"v8\""
       ]
     },
     {
       "Label": "bot_update",
       "Status": "InfraFailure",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/bot_update/0/stdout"
+          "URL": "/swarming/task/build-exception/steps/bot_update/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "json.output (invalid)",
-            "URL": "foo/steps/bot_update/0/logs/json.output__invalid_/0"
+            "URL": "/swarming/task/build-exception/steps/bot_update/0/logs/json.output__invalid_/0"
           }
         ]
       ],
       "Started": "2016-03-14T10:00:05Z",
       "Finished": "2016-03-14T10:00:06Z",
       "Duration": 1000000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 2,
       "Text": null
     },
     {
       "Label": "Uncaught Exception",
       "Status": "InfraFailure",
       "MainLink": [
         {
           "Label": "stdout",
-          "URL": "foo/steps/Uncaught_Exception/0/stdout"
+          "URL": "/swarming/task/build-exception/steps/Uncaught_Exception/0/stdout"
         }
       ],
       "SubLink": [
         [
           {
             "Label": "exception",
-            "URL": "foo/steps/Uncaught_Exception/0/logs/exception/0"
+            "URL": "/swarming/task/build-exception/steps/Uncaught_Exception/0/logs/exception/0"
           }
         ]
       ],
       "Started": "2016-03-14T10:00:07Z",
       "Finished": "2016-03-14T10:00:08Z",
       "Duration": 1000000000,
       "Type": "Step",
       "LevelsDeep": 0,
       "Verbosity": 2,
       "Text": null
@@ skipping 42 matching lines @@
       "Property": [
         {
           "Key": "Exit Code",
           "Value": "0"
         }
       ]
     }
   ],
   "Blame": null
 }