Add year to the date of the meetings

[carolromero]

🎩 User Story

If a process occurs between one year and another, it is important to indicate the year in the date of the meetings, to avoid confusion to the user.

🎯 Acceptance criteria

As a user I can see the year in the date of the meetings.

[mrcasals]

We should define a new date/datetime format, something like decidim_short, so that it includes the year:

http://guides.rubyonrails.org/i18n.html#adding-date-time-formats