Remove Binary Choice for Channels

[chrstinalin]

Relates to mozilla/addons#14855

Description

Minor tweaks throughout addons-server to keep channel use consistent.

Testing

1. Devhub landing page

• Badges shown as expected when add-on has a) versions in more than one channel, and b) enterprise version(s).

2. Reviewer's review log, developer profile review-enterprise link

• Log should only be visible with correct permissions.
• Log correctly links to the review-enterprise review page.

3. ./manage.py process_addons --channel arg

• Should be able to run filtered on enterprise channel.
• --help shows the enterprise channel as an option.

Should generally cover the hanging threads related to channels that did not account for enterprise channels.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).