[enh] New getters on MEP model, integrating with TimePeriod manager

Until now, we had many getter property like:

    MEP.current_organizations
    MEP.old_organizations
    etc...

This lead to duplicate code.
We fix this by using newly introduced TimePeriod manager and queryset
classes. The old methods are now replaced with simple getters:

    MEP.get_organizations

(The get_ prefix is added to avoid overwriting the original field..)

We can use them with the manager helper methods like this:

    MEP.get_organizations.only_current()
    MEP.get_organizations.only_old()
[enh] Update MEP templates to use the new getter methods

Also make some changes to display the MEP's historical infos which was
reintroduced recently in Parltack.