Updated tips & donations default suggested value

[kevinansfield]

closes https://linear.app/tryghost/issue/PLG-156

• updated all default fixtures to use 500 ($5) as the default suggested donation value
• added migration to update existing settings using the old default of 0 to 500
  • this is fine to apply because the feature hasn't been released so there's no explicit 0 values in the wild
• added an acceptance test for the adminx-settings tips & donations section

[github-actions]

It looks like this PR contains a migration 👀
Here's the checklist for reviewing migrations:

General requirements

• Satisfies idempotency requirement (both up() and down())
• Does not reference models
• Filename is in the correct format (and correctly ordered)
• Targets the next minor version
• All code paths have appropriate log messages
• Uses the correct utils
• Contains a minimal changeset
• Does not mix DDL/DML operations
• Tested in MySQL and SQLite

Schema changes

• Both schema change and related migration have been implemented
• For index changes: has been performance tested for large tables
• For new tables/columns: fields use the appropriate predefined field lengths
• For new tables/columns: field names follow the appropriate conventions
• Does not drop a non-alpha table outside of a major version

Data changes

• Mass updates/inserts are batched appropriately
• Does not loop over large tables/datasets
• Defends against missing or invalid data
• For settings updates: follows the appropriate guidelines

[sagzy]

LGTM 👍