Implement File upload step and data parsing 🚚

[rebecarubio]

Description

A user should be able to upload a file

See epic #1595 for context.

Relevant Job Stories

• When I import data from excel, I want Zetkin to filter out excel internal data columns from the columns I am presented with, so that I don’t have to try to understand if it is relevant data

Estimated size (S, M, L)

I think this issue is {L} because it takes care of different actions that we don't have defined and it's a new feature

Requirements:

[] Upload file clicking button
[] Upload file dragging file

Possible implementations

Reference https://github.com/zetkin/organize.zetk.in/blob/master/src/actions/importer.js

Design specifications

Open questions

[ziggabyte]

Closed as completed by #1605 and #1612