Automate base64 String Creation For DQL Questions

Description

Currently, we have a function that can provide base64 strings for the different database files that we use. This approach is very inflexible. In this issue, we need to automatically create the base64 string. We need to access the database file anyway, so we can simply add a step, during which we base64 encode the database file and then we add this to the end of the moodle-xml.

Todo

• remove old functionality for adding base64 strings to moodle-xml
• automatically, and ad-hoc, generate base64 strings for the database files that are provided in the YAML file (this entails making providing the database mandatory; also, we need to rethink how the database is provided (e.g. string vs path))
• add tests for the new functionality
• add doc strings for the new functionality (to support #13 (closed))
• make sure that the generated moodle-xml files are compatible with ISIS (for now, it is sufficient, if we generate the exact same moodle-xml files, that the current approach generates)