normalize/tidy up db schema

[g-k]

see also: #115

psiinon and I ran into an issue with psql needing us to quote mixed case column names e.g.

dependency_observatory=# select reports.npmsio_score, reports."directVulnsCritical_score" FROM reports;

proposed changes (can be separate PRs):

• [ ] rename mixed case columns to lower case
• [ ] define naming conventions for indexes and constraints https://docs.sqlalchemy.org/en/13/core/constraints.html#constraint-naming-conventions
• [ ] rename DB table names to match model names e.g. reports?
  • [ ] rename columns to be shorter
• [ ] write up the conventions somewhere
• [ ] dump a DB schema to docs (e.g. with https://www.schemacrawler.com/index.html , https://dbeaver.io/ , https://wiki.postgresql.org/wiki/Design_Tools#Commandline_Tools
• [ ] use constraints? https://www.postgresql.org/docs/9.4/ddl-constraints.html
• [ ] use mapped expressions? https://docs.sqlalchemy.org/en/13/orm/mapped_sql_expr.html

[g-k]

Rename reports.authors to maintainers so that's consistent throughout (or find an ecosystem agnostic term).