[WIP] SE-1321 add multiple files upload allowed option

Description

This PR adds an option to toggle between allowing multiple files uploaded and only allowing a single file. This is fully backwards compatible. Currently behaviour is to allow students to select an arbitrary number of files for the file upload response. Adding this option allows course authors to limit this to a single file upload.

*JIRA tickets*: TBD

*Dependencies*: None

*Screenshots*:

![1564542275](https://user-images.githubusercontent.com/9714796/62180562-9c91b200-b38f-11e9-8261-76b731123306.png)

*Sandbox URL*: TBD

*Merge deadline*: None

*Testing instructions*:

TBD

*Author notes and concerns*:

*Reviewers*

  • [ ] @pomegranited

  • [ ] edX reviewer[s] TBD

Status

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Samuel Walladge

Repo

edx/edx-ora2

Customer

Epic Link

None

OSCM Assignee

None

Priority

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