Expose more ENV variables for S3 Deployment

[skrawcz]

Is your feature request related to a problem? Please describe.
There are too many hardcoded paths with the S3 deployment.
Server root: /root/.burr_server
ORM files are relative off of the root
DB file name is fixed to be db.sqlite3

Describe the solution you'd like
Expose following ENV variables:

BURR_SERVER_ROOT
BURR_ORM_MIGRATION_LOCATION
BURR_DB_FILE_NAME

Describe alternatives you've considered
N/A

Additional context
This is from discord thread.

[elijahbenizzy]

BURR_ORM_MIGRATION_LOCATION should come from aerich init-db --location, right?

[skrawcz]

BURR_ORM_MIGRATION_LOCATION should come from aerich init-db --location, right?

See what script in #524 would become.

[elijahbenizzy]

BURR_ORM_MIGRATION_LOCATION should come from aerich init-db --location, right?

See what script in #524 would become.

The equivalent for local dev is on the PR that's still in draft