Allow migrate_drupal_ui source database to be set in settings.php

Problem/Motivation

When running and debugging migrations from Drupal 7, it becomes tedious and frustrating to have to repeatedly set the database connection up.

Proposed resolution

Allow the fields on the Credential form, the database connection and the source base paths, to be set up via settings. Suggested key names are:
migrate - for the source database. This key is the fallback key used by the migrate api.
migrate_source_connection - The database key for the source database connection.
migrate_d6_file_public_path - The source base path for Drupal6 public file
migrate_file_public_path- The source base path for public files
migrate_file_private_path - The source base path for private files

The original proposal was to allow just the source database to be set up via settings by looking for a database with the key name migrate_drupal_source. If it is present, skip the "Source database" section of migrate_drupal_ui's CredentialForm.

Before
Before.
After
For these screenshots the following were in settings.local.php

$settings['migrate_source_version'] = '6';
$settings['migrate_file_public_path'] = '/var/www/html/drupal/sites/default/d6files';
$settings['migrate_file_private_path'] = '/var/www/html/private';

Select from database array
No migrate_source_connection defined

Remaining tasks

• Reroll (Novice)
• Agree on key names, migrate, migrate_file_public_path, migrate_file_private_path
• Usability review
• Manual testing. Done in #77
• Review

User interface changes

If a database is already set up, the "Source database" section of Drupal\migrate_drupal_ui\Form\CredentialForm is omitted.

API changes

Data model changes

None.

Release notes snippet

TBD