Attach files to tasks

[chrl]

As a new feature request, implement file uploads and attachments to tasks.

[tarlepp]

This is a good feature. I think that files should be able to attach to all objects (project, sprint, story, task, etc.)

[opensaurusrex]

There should also be a way to configure how they are stored, aws being one of them.

[tarlepp]

This is going to be on next version of Taskboard, model for this is already done (https://github.com/tarlepp/Taskboard/blob/angular/backend/api/models/File.js).

Actual storage configuration need more thinking to do.