Fix of admin_links for Django 2

[zbohm]

Hi, here I offer some fixies for Django 2. The most serious is fix of function admin_links:

• Html code from function admin_links must be marked as SafeText.
  • Replace <br> to container <div>. Tag BR is unpaired tag. It is better to use a container, for examle for css styling.
• 'django.contrib.messages', in INSTALL_APP is required by Django 2.2.
• I suggest to restrict Django versions to django >= 1.11, < 2.2.99.