Add a wiki_link field when adding a meeting.

This field is then added to the description of the meeting. This might be redundant but
emphasis to the user the fact that he/she might have to fill in a wiki page.

We check that the link isn't present in the description before adding it.

Fixes https://fedorahosted.org/fedocal/ticket/77
Fix adding the link as a link using the markdown syntax
Specify that the foreign_key are `onupdate="cascade"`
Fix indentation
Merge pull request #87 from fedora-infra/add_url

Add url