Reorganize db-functions out of general content-building components

[thomaseleff]

Currently, db-functions are located in different levels of the assemblit library (users-db functions in ~/vault.py, sessions-db functions in ~/components/_selector.py, etc.). Unlike generic content building functions, database management functions are specific to each database, given the different dependencies each database has with other databases.

To Do:

• Refactor the db-functions out of the current modules, creating new modules for each database that contain standard add, update, delete functions
• For each database, include a standard schema object that describes the table columns and data-types
• Ensure all database methods properly close the database

[thomaseleff]

• Remove assemblit.setup from the imports of assemblit.database.generic.
• Remove all references to assemblit.database.generic within all modules, classes and functions in the assemblit.pages namespace

[thomaseleff]

For the users database,

• Move assemblit.vault into assemblit.auth