* use more convenient testing functions
* do not display + sign with 0% change in emails
* Rename module/file/function names
before, `weekly_report` was also used for monthly reports and that was a
bit confusing to read in code.
* Refactor send_email_report.ex
This commit improves readability by refactoring the code into smaller
functions and reducing the number of arguments given to functions.
But more importantly, it stops making duplicate stats queries for every
email recipient by moving the queries out of the for loop.
* Refactor: move querying logic out of the worker module
and merge all stats information under a single `stats` assign.
