Consistent use of capitalization across queries

[bram-vdberg]

Background
In PR #10 there was a discussion about the inconsistency in SQL keyword capitalization across our queries. @fhenneke suggested enforcing this through linting checks. In order to enforce this we would also need to fix the capitalization in all queries to ensure they are consistent.

Proposed Solution

1. Implement lowercase SQL syntax across all queries
2. Add a linting check to enforce this in the CI:

[sqlfluff:rules:capitalisation.keywords]
capitalisation_policy = lower

[fleupold]

Sounds reasonable!