Redundant and confuse date handling

The following antipatterns are chosen regarding the date argument of many get_data() methods:

• repeating exactly the same code
• requirement to explicitly specify accepts_date
• having a date argument in get_data() methods, even they don't accept dates
• output: "[INFO] No date specified. Getting data for yesterday" even it doesn't accept a date; this suggests it accepts a date
• in gitlab_ci_jobs.py: not accepting date, defaulting date argument with None, but conditionally overwrite this value with yesterday and using this value; condition is always true
• redundant convert_dates() method
• redundant and inelegant date to datetime conversion in gitlab_merge_requests.py