Move upload source step after details in submission flow

[willdurand]

Fixes mozilla/addons#16133

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In addition to changing the order of steps, I had to update a message. Indeed, we currently show a "remember" message conditionally in the "details" step:

In this patch, I replaced it with a different message since the source code is uploaded after:

[eviljeff]

r+wc about copy change

[abyrne-moz]

Given that the add-on builder tries to build the code, it seems like they might not need to provide instructions anymore, so this feels acceptable to me.

[wagnerand-moz]

r+ for the updated copy about providing source code building instructions.

[wagnerand-moz]

Given that the add-on builder tries to build the code, it seems like they might not need to provide instructions anymore, so this feels acceptable to me.

Once the builder is rolled out we probably need to change this and other copy as well, both on AMO and in other places (e.g. extension workshop). Instructions still need to be provided, but in a very different form. We have some ideas but haven't talked in depth yet what that would look like. One requirement we already know today is that they need to be machine-readable, so it could be as simple as a rpm run target we define.