add Unlock to the Operations menu on the main Feeds page

Problem/Motivation

When a Feed breaks, there isn't always enough feedback for the user to know that the feed is locked and how to unlock it.

Steps to reproduce

Create a Feed Type.
Create a Feed.
Run a bad import (maybe with a buggy preprocessor hook) that breaks the feed.

Result: the Operations dropdown contains only Edit (and Devel, if installed), and it's difficult for the user to know why the Import options have disappeared. Similarly, the Edit page for the Feed only has Save, not Save and Import, but again, there's no Unlock link on this page, so the user can't tell why the Import option is gone.

Proposed resolution

When a Feed has become locked, add an Unlock option to the Operations dropdown (and any other screens where Unlock does not currently appear, like the Feed's edit page at https://example.org/feed/4/edit )

This is related to Damaged Import Process. Like that user, I was mystified when one of my feeds suddenly stopped having an Import option, when the others all had one. I tried several approaches to try to get it working, and it's a difficult issue to search on. (The solution on that issue turned out to be something else, but that was the only mention I found of locked Feeds, and the only info I found that helped me unlock my own locked Feed.)

This relatively simple change to the UI could save a lot of time for other users who end up with a locked feed.

Thank you!