Demo site improvements

[thibaudcolas]

Changes to the demo site to make it easier to review PRs like #97 and #90.

• Make manage.py executable
• Change tests database name so it does not conflict with wagtailmedia folder (this prevented the demo site from running with a basic virtualenv setup).
• Allow all hosts for demo site
• Add basic page template for demo site’s test page (useful to test the rendering of media like in Embed media in rich text fields using Draftail #90)
• Set up Wagtail document URLs in demo site. This just makes it easier to compare the media chooser with Wagtail’s built-in choosers.

[thibaudcolas]

Thank you both 🤘