As part of #573, we want to make sure that it's possible to have the
`dmd-web` app load the database as read-only to avoid arbitrary
executions of modifications of the database.

This simplifies things by introducing a new method for opening the
database - which can be used centrally in the future - which more safely
modifies our

Note that we need to prefix the database URL with `file:` for our SQLite
driver to correctly pick up the querystring parameters.