Refactor the saved links display metabox

[benlk]

Changes

• cloned Wordpress' WP_List_Table class file to inc/saved-links and renamed its class to clone_WP_List_Table. The reason for cloning this and not extending it is because WP_List_Table is a private, unstable class that developerssay should not be directly extended.
• Extended clone_WP_List_Table to create Saved_Links_List_Table with rendering options, columns, and a query specific for the lroundups-links posts.
• Changed the saved links display page to an instance of Saved_Links_List_Table
• Changed the "Send to editor" button and other other JavaScripts to reflect changed selectors.
• Added a spinner to the "Filter" button to indicate that the links are loading.
• CSS for miscellaneous pieces, for uniform looks
  • thead td and thead td checkbox so they appear the same as the inputs in the tbody
  • the "send to editor" button, for vertical centering
  • remove floats on the select in the table nav, because why would you want that anyways?
• inline docs

Why

To hopefully make this more maintainable.

For #63.

[benlk]

Do we want tests for this?

[rnagle]

@benlk Yes, please, to tests.

Also, in testing this, I discovered a bug when trying to publish a link roundup. After clicking the "publish" button, the page redirects to a "Are you sure you want to do this?" message. The link roundup post remains a draft when I navigate back to the post list page in the dashboard.

[benlk]

I'm going to close this PR until this is mergeable, because the WIP tests are breaking actual functionality. The file isn't testable unless the WordPress admin isn't loaded, because the WordPress admin when loaded defines ABSPATH and the testing suite has already defined that. :/