Add a non-archetype DatabasePlugin

[markschaake]

We now have several projects that depend on both slick and flyway for database integration and database migration, respectively. This means we have duplicate boilerplate across project build files. The solution: create a plugin of course!

The DatabasePlugin should not be an archetype, but instead a "mixin" plugin. Usage would be something like this:

// in build.sbt
enablePlugins(WebappPlugin, DatabasePlugin)

DatabasePlugin Spec

The DatabasePlugin should:

• add slick as a library dependency (which version - 2.1 or 3.0? I think 3.0 even though it was just released)
• add flyway as a build dependency
• add flyway settings to the build
• apply conventions (such as migration directory)
• provide a task to generate slick tables from a connected db (we have done this, for example, here: https://github.com/allenai/scholar/blob/e0e21bb594e93324e41547773778bdd17b6cf695/data/sources/arxiv/slick-codegen.sbt)

As a reference for necessary flyway settings, you can look here: https://github.com/allenai/scholar/blob/e0e21bb594e93324e41547773778bdd17b6cf695/data/crawler/flyway.sbt