Allow more customization for other conventions

[queengooborg]

Regardless of the setting of the APP_NAME, the login page always says "BLFC Volunteer Check-In" and the logo remains the BLFC chip. It would probably be good to allow customization of the branding via environment variables!

[Gawdl3y]

Since #38 was merged, a lot of this is addressed. As things stand, the logo will still have to be manually replaced in the files, sadly. I've been thinking about making it possible to upload an image to Tracker in the admin panel to use as the logo to simplify this. It may also be worth adding a basic "convention name" setting or something like that to use for branding a few more things than just the APP_NAME env var does.