[META] Build a Feeds migrate UI (mockups) [#2986955]

Problem/Motivation

Let's decide on a UI.

Proposed resolution

Remaining tasks

User interface changes

Here is the UI work in progress: https://docs.google.com/presentation/d/1J2Ab47q41cD0Q6pIk_72gd7yxaQTqeJiflqVTEKJt38/edit#slide=id.g36fb9780b4_0_5

During our weekly feeds meetup we discussed the highest priority part of the UI which will allow devs to start coding is the for with which you can "add" a process plugin, similar to when you add a Tamper plugin for Feeds.

If we can get a UI for this, the devs will have a place they can hook the processor plugins up to.

@megachriz mentions:

This is challenging because there is no finished mapping screen yet in Feeeds Migrate. So it's harder to come up with a list where you could put 'add process plugins' links next to.

Process plugins need at least a source and a target to operate.

If we could find a way to parse existing mappings from a Migration entity, we could just build a list of what is already there in the Migration entity. We could also on the add form require the user to specify the source and target, as a temporary solution. In a later phase we would then require users to define mappings first and then add process plugins to it.

We might as well just keep the UI as in Feeds and have a separate tab for adding/configuring process plugins. In this case adding a mapping would mean just adding a process plugin of type ‘get’.

API changes

A challenge will be that when you are going to add other process plugins to that mapping, the process plugin ‘get’ will need to vanish. And when you decide to remove process plugins again, the process plugin of type ‘get’ will need to return in order to keep the mapping.

May be worth creating a new issue for this challenge at some point.