[META] - Migrate UI Roadmap

I think this module should take responsibility for editing, building and creating migrations. We'll provide a UI on-top of migrations themselves so they can be manipulated but also a UI on top of migration templates so they can be configured and edited also. Once you've constructed your migrations, it will be onto using a "runner" like the ones provided in migrate_tools, migrate_manifest or Migrate Wizard once that comes about.

This module is now called migrate_ui - #2624062: Decide on the module name before publishing to a full project

What's already done

1. View a list of the migrations on your website
2. Edit those migrations, including tags, migration name, process mapping and advanced process plugin configuration via the UI
3. Delete migrations
4. Create new migrations entirely from scratch
5. Add some filters to the list view - #2624132: Add filters to the listing page
6. Allow the process pipeline to be re-ordered - #2624058: Add a way to re-order process steps
7. Pickup other modules process plugins - #2624128: Allow other modules process plugins to be picked up.
8. Migrate UI Templates - #2626464: Add a UI for Migration Templates

What's Next

1. Find a way to build advanced migrations - #2629340: Support migrations that need advanced building
2. Find a way to "configure" sources. Helps with builders but also any runner must solve this.