Description of problem:
The subscription details page and add subscription page should render breadcrumb bars in the header as seen in similar pages (like Module Stream details). Instead, no headers are rendered at all.

Version-Release number of selected component (if applicable):
n/a

How reproducible:
Always

Steps to Reproduce:
1. Open Content -> Subscriptions -> <any subscription>.
2. Open Content -> Subscriptions -> Add Subscription.

Actual results:
The headers will be missing and the only way to return to the subscription list is to use the browser back button.

Expected results:
Breadcrumb bars should be displayed showing the current path.

Additional info:
This breadcrumb bar exists in 'webpack/scenes/Subscriptions/Details/SubscriptionDetails.js' and 'webpack/scenes/Subscriptions/UpstreamSubscriptions/UpstreamSubscriptionsPage.js' but are unrendered for some reason. Needs further investigation.