Merge `wp_handle_upload()` and `wp_handle_sideload()` by making them each wrap a new function: `_wp_handle_upload()`.

Props DrewAPicture for docs.
Fixes #23686.

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Twenty Fourteen: Remove unused variable after [27120].

props michalzuber.
fixes #28928.
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TinyMCE: include https://github.com/tinymce/tinymce/commit/709266e308afb89fdfc671b8ef4cfc071790881e that fixes the UI state updates for the "link" button. See #28576.
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Media Grid:

Instead of a separate bulk selection mode, persistently show a checkbox for each item. Restores the more familiar bulk actions dropdown + action button.

props ericlewis. see #28842.


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Fix jshint errors introduced in [29212]. see #28842.

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TinyMCE: add the 'colorpicker' and 'lists' plugins, fixes #28939, #27359.
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Declare the variable before using it.

props danielbachhuber.
fixes #28932.
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Don't 404 for empty feeds.

fixes #18505.
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