fix(mrf): attachment equality validation

[justynoh]

Problem

Attachment equality validation had to be removed previously due to backward compatibility issues in v6.119.1. This PR restores it.

Solution

Inject a new md5Hash key in the database for attachment answers.

• The key is populated with the attachment MD5 hash after virus scanning has been completed.
• The incoming attachment's MD5 is compared with the existing attachment's MD5 in order to determine equality.

At the same time, I also made a change to the frontend mutation name that was bothering me. paiseh

Breaking Changes

• No - this PR is backwards compatible

Tests

• Create an MRF with two attachment fields. Respondent 1 can edit question 1. Respondent 2 can edit question 2.

Backward-compatibility tests

• Submit an MRF with an attachment 1 on the old application version as respondent 1.
• Now, submit the same form with an attachment 2 on the new application version as respondent 2. This should succeed.

Tests

• Submit an MRF with an attachment 1 on the new application version as respondent 1.
• Now, submit the same form with an attachment 2 on the new application version as respondent 2. This should succeed.

[kathleenkhy]

lgtm!