Explain how to use the database schema during application startup

[mkurz]

The section "Using Ebean during application startup" (#910) said that depending on ApplicationEvolutions doesn't help to use the database schema while components get created, as in dev mode it doesn't wait for pending evolutions. That's true, but incomplete: a component that depends on ApplicationEvolutions gets created after Play checked (and, with autoApply, applied) the evolutions, so it can use the schema if upToDate() returns true. In dev mode, applying the evolutions in the browser reloads the application, which creates the component again. As ApplicationEvolutions depends on DynamicEvolutions, the Ebean databases exist as well.

This replaces that sentence with a sample of that pattern, and keeps letting Play apply evolutions automatically in dev mode as an alternative, including the warning about downs.

Play's docs now describe this pattern as well (playframework/playframework#14339). That section isn't linked here, as so far it's only in the docs of Play's main, not in the docs of the Play versions that 8.x and 7.x use.

Testing

• The new sample gets compiled by the docs build, and a new test in JavaEbeanTest creates it in test mode, where Play applies evolutions automatically, and checks that it inserted its initial data.
• cd docs && sbt "validateCode; test" passes.

References

• Cannot create and properly instantiate an eagerly() bound DI component when dependent on Ebean and Evolutions running. #65
• Document accessing the database while the application starts playframework#14339

Backports: #914 (8.x), #915 (7.x)

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