[IMPR] Reduce code complexity of welcome.py
[IMPR] Reduce code complexity of welcome.py

- split main function into parts to reduce code complexity
  reported by codeclimate as "critical"
- remove issue_deprecation_warning which is 3 years old and replace it with
  a simple warning of unknown option.

Change-Id: I4c23bf754872ba48c496862104bfe48aa37a1e94

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core