Decide what to do about important contextual links when that module is disabled or restricted

Follow up from #1874664-110: Reconcile "Edit" toolbar option with local tasks (tabs) and contextual links for editing. In that issue, we removed "Edit" as a primary tab (MENU_CONTEXT_PAGE) from node pages, and instead made it a contextual link (which it already was on all pages where a node is rendered other than its primary page) on the primary node page as well.

That has repercussions though:

1. What if you disable contextual.module? Then, there's no link to edit a node from its view page. I suggested in #1874664-112: Reconcile "Edit" toolbar option with local tasks (tabs) and contextual links for editing that that's ok, because you can still edit it from admin/content. After all, a link to edit a node from its view page is a "contextual" link (at least in the broad sense of the word "contextual", even if not in the narrower D7 sense), so losing that convenience when you disable the module shouldn't be a surprise.
2. What about non-admins without "access contextual links" permission? I also suggested in that same comment that we remove that permission (similarly to how we don't have a "access local tasks" permission), but #1874664-111: Reconcile "Edit" toolbar option with local tasks (tabs) and contextual links for editing lists some reasons why it may be desirable to have roles without access to contextual links, at least the way we currently render them.
3. What about non-admins when contextual.module is disabled? This is an extension of the first point, but for people who don't have admin/content as an alternate. Perhaps we can change that listing page to a View that everyone has access to, but one that is filtered to only those items you can edit?

Or, maybe the responses to the above items are not good, and we need a fallback mechanism for important contextual links to automatically become primary tabs for users who can't access contextual links?

While the specific use case currently is the node's Edit link, it's possible that more items should move from tabs to contextual links, so per the issue title, let's evaluate with that broader perspective.