Add created/modified fields for Project and Description

* Modification or creation of Description or Download updates Project (field: modified)
* Sitemap takes into account modified field

SQL queries:
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ALTER TABLE `projects_project` ADD COLUMN `modified` datetime NOT NULL AFTER `category_id`;
ALTER TABLE `projects_project` ADD COLUMN `created` datetime NOT NULL AFTER `category_id`;
ALTER TABLE `projects_description` ADD COLUMN `modified` datetime NOT NULL AFTER `data_anchor`;
ALTER TABLE `projects_description` ADD COLUMN `created` datetime NOT NULL AFTER `data_anchor`;

Django shell:
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from projects.models import *
from django.utils import timezone

for p in Project.objects.all():
    p.created = timezone.now()
    p.save()

for d in Description.objects.all():
    d.created = timezone.now()
    d.save()