Provide a way to check specific links immediately [#3616064]

Problem/Motivation

It's currently not possible to check specific links immediately.

Proposed resolution

1. Add an Action plugin so that multiple links can be checked directly via the admin UI.
2. Add an operation so that individual links can be checked.
3. Add a drush command so that you can index specific links

Remaining tasks

~~Provide MR.~~

User interface changes

There's now new operations and actions for checking links.

API changes

N/A

Data model changes

N/A

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3616064-check-links-directly changes, plain diff MR !161
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