Fix/systemd pamac-mirrorlist timer and service

Created by: kewlfft

• This is adding pacman -Syy post command (see below).
• Also adding randomized delay to avoid traffic at 0:00 from everybody.
• Increasing the frequency to every 2 weeks should be reasonable as -f5 is quick and the randomization ensure this is not concentrated at the same time.

It cannot be stressed enough, that every run of pacman-mirrors requires you to syncronize your database
$ sudo pacman -Syy
If you fail to do so, the issue/s which had you make change might not be solved. Furthermore you might run into partial-updated scenario which can cause havoc in your system.