Rename environment variables to semantic strings

[eternaltyro]

Rename existing arbitrary environment variables to semantic strings that are self-explanatory. There are some strings that are expected to be exactly the same when passed as ENVVARS. We can leave them be, for all other env variables that we chose during the course of development, there can be improvements made.

OLD ENV KEY	NEW ENV KEY	Description
EMAIL_HOST	SMTP_HOST	SMTP Host
EMAIL_HOST_USER	SMTP_USER	SMTP Username
EMAIL_HOST_PASSWORD	SMTP_PASSWORD	SMTP Password
REPLY_TO_EMAIL	SMTP_REPLY_TO_ADDRESS	Reply-to address
OSM_API_KEY	OSM_OAUTH1_APP_CONSUMER_KEY	OAuth1 API Key
OSM_API_SECRET	OSM_OAUTH1_APP_CONSUMER_SECRET	OAuth1 API Secret
EXPORT_DOWNLOAD_ROOT	EXPORTS_DATA_DIR	Directory on disk where exports data is stored
EXPORT_STAGING_ROOT	EXPORTS_DATA_STAGING_TEMP_DIR	Temporary directory where exports data is staged before being copied elsewhere

This is my suggestion.

[eternaltyro]

PR that initiated this request: #498