Allow choosing the text format for each footnote

Problem/Motivation

I need multiple footnote styles, sometimes within the same node. Some footnotes should be plain text, and not even be wrapped in a <p> tag. Other footnotes are more complex, and need to allow for multiple paragraphs, images, lists, etc.

Steps to reproduce

Create a footnote. You can't select the text format, as it is hidden in the modal (the `footnote` format that ships with the module is always used).

Proposed resolution

Include a data-format="" attribute that defines the text format for each footnote. If omitted, use the "footnote" format for backward-compatibility.

Remaining tasks

User interface changes

API changes

Data model changes