Added custom filter module in shared app and pretty_datetime filter

Added a custom filter module in shared app and registered new filter "pretty_datetime", which takes an input and returns a formatted version of date-time data for easy readability. Expected input is a string, but @stringfilter decorator was added to ensure all input is converted to string to reduce changes of errors. Filter is put behind a try/except/pass block to make sure no security vulnerabilities introduced in the case of Exception. Updated issue_detail template with pretty_datetime filters on returned ISO 8601 datetimes from python-gitlab.