ref: migrate inline queries for retrieval of submissions metadata to model static methods

[karrui]

Problem

This PR extracts out the inlined mongoose queries in the /:formId([a-fA-F0-9]{24})/adminform/submissions/metadata controller handler. Also adds celebrate middleware validation for the route.

Solution

Features:

• Add EncryptSubmissionModel#findSingleMetadata static method to retrieve metadata for a single submission
• Add EncryptSubmissionModel#findAllMetadatabyFormId static method to retrieve all submission metadatas for the given form id.
• Add celebrate middleware validation to /:formId([a-fA-F0-9]{24})/adminform/submissions/metadata endpoint.

Improvements:

• Replace inlined queries in the controller handler for /:formId([a-fA-F0-9]{24})/adminform/submissions/metadata endpoint with EncryptSubmissionModel static methods.