Use months and not weeks to display donations

[Zughy]

Hi there, longtime user of Liberapay, so first of all, thanks for what you're doing!

I think that thinking in weeks is counterproductive, as people (due to salaries I suppose) are used to think in months. Off the top of my head I can't name one service I know that thinks in weeks. Moreover, this creates two problems:

1. noise in the donation page. This
   
   is less clear than this
   
2. it forces people to do the manual conversion when they see the current donation amount of someone, as we think in months. "€3.5 per week? Ok so in months that's..."

[Changaco]

The last time something like this was discussed (liberapay/salon#206) it failed to reach consensus.