feat(app): db config updated: properties removed from persistence.xml

This commit prevents sensitive data from being occasionally commited to
repo.
feat(app): liquibase added
chore(app): .gitignore updated
chore(codestyle): app initializer renamed to db initializer

This is the only function that this class is doing.
feat(app): signup page changed
chore(app): dart version updated to 0.9.6
chore(app): login page reformatted
feat(app): liquibase changed to flyway