Calculate the longest mandate for each ministry and show it as a special tag

[sandriu]

On dignitas.ro, since we have a pretty nice collection of ministerial mandates (the exact time intervals for each minister, each mandate), we could calculate the longest (continuous) mandate for each minister and create a special tag for it. The idea is that the longer the mandate for a minister, the more achievements that minister should have (thanks to a longer period of time for implementing various new or ongoing projects).

TBD: how and where this special tag should be displayed on a minister's profile page: below the thumbnail photo or maybe on the list of his/her Relationships.