Insert attachments into the text editor using the new media modal

[koopersmith]

We have a new media modal at #21390, but it's not particularly useful unless it integrates into our existing UI. Let's make it work for inserting attachments into the text editor.

Related: #10161

In [22036]:

BUTTON.

Add a "Beta Media" button to the post editor. Currently, it is only capable of inserting images. Other attachment types and galleries need not apply... yet.

• Added wp.media.string.image( attachment, props ) for generating an image as a string from an attachment and relevant attachment display properties.
• Properly localized the gallery workflow.
• Added Workflow.update(), which closes the modal, triggers an update event, and resets the selection.
• Added wp.mce.media to manage the various media workflows for editors.

see #21813, #21814, #21390.

This needs ​http://f.cl.ly/items/0R3l3241131I0Z0A3E0G/attachment-details.png. Normally, this screen will be bypassed for the visual editor as you can edit it using the in-editor view, but we need it for a textarea insert.

In [22247]:

Add attachment display settings to the media modal.

• Add a media view for button groups.
• Add button dropdown containers.
• Add a dropdown property to the button media model.

fixes #22206, #21814, see #21390, #21813, #21598.