Document application architecture

[emkll]

As the application complexity increases, it might be useful to document (for example using a diagram) the high-level application architecture to better understand the different major components at a glance and how they interact with each other.

This will help both developers and newcomers, especially those not familiar with Qt and Qt-specific patterns.

[eloquence]

@redshiftzero and @creviera have been working on a GDoc with some initial notes; for the 7/24-8/7 document, we've committed to aim to clean up this document to a degree that it can readily be turned into a first docs PR. This will not include any architectural diagrams, just yet.

[sssoleileraaa]

This is where we're keeping the WIP-architecture docs: https://github.com/freedomofpress/securedrop-client/wiki/Architecture, which right now is very minimal and hopefully will steadily grow and become more detailed as we devote a bit of time to them each week and build out the client.

[kushaldas]

I am approving the current document. We have to remember to keep updating it for any architectural change. @creviera Thank you for this.