"Escape" caption field by removing quotes from caption content.

[goldenapples]

Due to known issues with WP's shortcode parser, attribute fields can't contain special characters like quote marks, square brackets, or HTML tags.

This means that the caption feature here is falling short of full feature parity with WordPress's image caption experience, which can hold quotes, tags, and special characters.

Until a solution is figured out, either in core or in Shortcake, there could be a bit of protection for users here. One possible solution is preventing quotes from being typed into these fields or transforming double quotes to single quotes.

[goldenapples]

The current state is functional when captions etc are inserted through the UI. However, when captions or alt text are inserted through the Media Editor, they don't get url-encoded before being passed to the editor:

[goldenapples]

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