How to specify internal disease normalizer environment?

Drug indication data requires a disease normalizer instance, so the CLI currently tries to identify a `disease_concepts` table location and update it if necessary. However, there are a few condition branches where various values aren't properly set, raising UnboundLocalErrors.

Right now we can use DISEASE_NORM_DB_URL to set a URL, and I think if you define it as a blank string it will push up to prod (and then use the environment set in DISEASE_NORM_ENV). You can also set THERAPY_NORM_PROD to prevent attempts to check/update the disease database. I think we just need to ensure documentation is updated (if only for my sake).

Oh, also, the CLI `_check_disease_normalizer` method needs to check against ChEMBL too, not just HemOnc.

Start with the CLI `_check_disease_normalizer` method and the handling of DISEASE_NORM_DB_URL, DISEASE_NORM_ENV, and THERAPY_NORM_PROD. Clarify the documentation for internal disease-normalizer environments, resolve the condition branches that can raise UnboundLocalError, and ensure the check covers both HemOnc and ChEMBL.

Written by the indexing model from the issue text.