Resolve "Make deployment script less HZDR-specific"

Closes #358 (closed)

TODO:

• try to get rid of poetry env use python3.7
• make config simpler and re-think "local execution"

Notes for development:

• LOCAL_EXECUTION setting renamed to DEVELOPMENT
• ssh key renamed from heliport_rsa_local_execution to heliport_rsa_development
• sqlite database renamed from db.sqlite3 to heliport.sqlite3
• some more settings cleanup needed; see #361 (closed)

Notes for deployment:

• DB settings now via env.db_url; already added in HELIPORT.env
• ALLOWED_HOSTS, USE_CELERY and REGISTER_HANDLES now explicit settings; already added in HELIPORT.env