Require fewer things in the MergeAdapter

[kookster]

The MergeAdapter requires a certain number of params, before the Observable will return. This causes the player to silently "load" forever when we can't find all the params.

We need to keep some of these required params. The purpose of the merge adapter is to make sure we don't leave the loading state too soon, when there are multiple data sources. For instance, if you specified the title in the query param ?tt=whatever, but also loaded the RSS feed for the rest of the data, we need to wait for that feed to load before rendering.

Several TODOs here:

• [ ] console.error from the MergeAdapter after X seconds pass and we're still missing a required parameter. (Long enough that we're sure the feed / draper have loaded).
• [ ] Only title and audioUrl are required.
• [ ] Render blank subtitle by default
• [ ] Have a default image in this codebase for the feedArtworkUrl. Also some sort of default background.
• [ ] Hide subscribe button if subscribeUrl is not present

[kookster]

not sure what is broken, assigning an estimate of 3

[cavis]

Original issue:

https://play.prx.org/e?tt=Testing%20Standalone&ts=test%20test%20test%20test&ui=%2F%2Fs3.amazonaws.com%2Fproduction.mediajoint.prx.org%2Fpublic%2Fcomatose_files%2F4625%2Fprx-logo_large.png&us=https:%2F%2Fbeta.prx.org%2Fstories%2F219680&ua=https:%2F%2Fcms.prx.org%2Fpub%2F989e07002296698426cb2fb2f64b7f9d%2F0%2Fweb%2Faudio_file%2F1529757%2Fbroadcast%2F278_The_Athletic_Brassiere.mp3