Add UI page to compare destination fields to process fields

Problem/Motivation

When importing to an entity with many destination fields, it becomes hard to verify if the field mapping in the 'process' section of the migration is complete.

We want to know about:

• Destination fields that are missing in the migration.
• "Orphan" process items ("pipelines"?) for non-existing destinations.

Steps to reproduce

Proposed resolution

Provide an overview page at "/admin/structure/migrate/manage/{migration_group}/migrations/{migration}/map".

Show a table with:
Columns:

• Description - same as in "Source" tab.
• Machine name - same as in "Source" tab.
• Source - same as in "Process" tab.
• Process plugin - same as in "Process" tab.
• Default - same as in "Process" tab.

Each table row is for one destination field and/or process field.

The table is split into sections.

1. Field names / keys where process and destination are present.
2. Field names / keys where the destination is missing.
3. Field names / keys where the process item is missing.
4. (optional) Field names / keys where the process item is missing, but which are in a special ignore-list.

Bonus:

• Allow migrations to specify a list of destination field names to ignore, so they don't distract when looking at the unmigrated destination fields.
• Find a way to inherit a huge part of this list from the migration group, or from a "parent" migration.

Remaining tasks

User interface changes

New overview page.

API changes

Data model changes