Improve multiple smallish things

[one-data-cookie]

Based on testing on dummy dbt project and Superset instance, as well as Slido staging environment several smaller things were caught:

• __init__.py:
  • Add missing space in help for exposures_path
• push_descriptions.py:
  • Run PUT request for updating descriptions only for necessary datasets
  • Add parsing by convert_markdown_to_plain_text for table descriptions
  • Check database columns against None, not empty string
• pull_dashboards.py:
  • Add empty exposure owner.email to final dict only, not through Superset dict
  • Force exposure name to be lowercase
• Improve logging, comments, and small details

Run times on slido-dbt-transformations and the replica of Slido Superset:

• pull_dashboards: 5m
• push_descriptions: 7m