Feature/refactor sources

[leo-mazzone]

Context

Changes proposed in this pull request

• Removed SourceWarehouse, and generalised code to deal with multiple types of warehouse (at least Postgres and SQLite)
• Removed datasets.toml and admin.py
• Improved and simplified interfaces of Sources, Selector, SourceColumn, Match, and moved these to matchbox.common.sources (out of matchbox.common.db)
• Sources now have an address, made up of their full name, and hashed representation of their warehouse
• Multiple sources can now be mapped to the same resolution
• Now storing column data in plain text on the server
• Moving client-side code out of server logic for query function
• Renamed some custom exceptions to clarify when they originate in the server
• Removed ability to query without specifying resolution when selecting from multiple tables
• Removed unused code in various places
• Automatically reformatted some docstrings

Guidance to review

• Further improvements to source columns are captured in MTCHBX-105
• I haven't changed it, but are we sure that upon indexing a dataset we want to update the value of the source in the backend on conflict?

Checklist:

• My code follows the style guidelines of this project
• New and existing unit tests pass locally with my changes

[wpfl-dbt]

Very well done!