Fix conference_logo.url Meta on a global scale.

[bojanpy]

Right now, I am adding the following to the pages I want to display the current conference image in the Meta:
pyconbalkan/speaker/views.py:67

            image=conference.conference_logo.url if conference.conference_logo else None

I would prefer if the image would be added always in the respective Meta, so that we don't need to insert this in every view.

Also this hardcoded image line should be removed then from pyconbalkan/core/templates/base.html:18:

    <meta name="og:image" content="https://pyconbalkan.com/static/img/pycon-logo.png"/>