[Preprint] Add 'Data Availability Statement' to model

Is your feature request related to a problem? Please describe.
Increasingly, open access publishers are requiring their authors to declare where their data is available publicly, and if it's not, to explain why. This is true for journals as well as preprints, though this request is being made specifically for the preprint-remodel branch.

Describe the solution you'd like
During the submission process, authors should be asked to make a selection from a dropdown menu of pre-configured choices in order to indicate where the data associated with their manuscript is available. Depending on the selection they make, they may need to provide additional information. The combination of the selection they make, and additional info would then generate a "Data Availability Statement" that will appear on public-facing Item pages.

Example:

Dropdown Selection Value -> Additional field -> Item page display

• Public repository -> URL -> The data associated with this paper is available at: [URL]
• Public repository: available after publication -> None -> The data associated with this paper will be made available after publication.
• Supplemental files -> None -> The data associated with this paper is available as supplemental files.
• Within the manuscript -> None -> The data associated with this paper is available within the manuscript.
• Available by request -> Contact Info -> The data associated with this paper is available upon request (linked)
• Managed by a third party -> Contact info -> The data associated with this paper is managed by a third party (linked)
• Not available -> Reason -> (display reason)

Describe alternatives you've considered
We currently make do by using the Additional Fields option to provide a text box on step 1 of the submission process. It alerts authors that they will have an opportunity to provide a link to their data in step 3. of the submission process, and asks them to provide a reason for not completing that step if they don't intend to. This process is a bit backward logically, and doesn't do much to encourage (or enforce) authors to share their data.

Additional context
Mock-up of potential revisions to Step 3. of the submission process attached.
Note that this mock-up also includes a proposal to separate 'Supplemental Files' from Data Availability statement, and suggests that Janeway ought to someday handle the upload and display of locally uploaded supplementary materials.

No files or tests are named. Start by tracing the preprint-remodel submission process and public-facing Item pages; define the dropdown choices, conditional fields, and generated Data Availability Statement, then verify each choice is captured during submission and displayed correctly.