Add multi database support for pending migration error

[beauraF]

Motivation / Background

PendingMigrationError actionable error currently does not support a multi database setup. The goal of this PR is to add support for multi database setup.

Detail

I have chosen to directly invoke the db:migrate rake task in the action, so any code change to the migrate task will be reflected. But I'm really unsure about this implementation, as it require to Rails.application.load_tasks and I can't find any similar pattern anywhere in the codebase.

Checklist

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.