Change homepage to use hard-coded file names

[mattstratton]

In order to process the images properly, we need to be able to match on them.

Right now, we allow any image name (relative to the event's source) to be set in that parameter.

I suggest instead we require it to be named logo_square.jpg and placed in that directory. We can add a boolean for event_logo which is set to default if they don't want to make one.

[bridgetkromhout]

I'm on board with this change.

[mattstratton]

Remind me, what is the workflow for a new event? They are prompted for this image, right?

[mattstratton]

This is probably more of a documentation fix in the README for how you set a square homepage logo.

[bridgetkromhout]

We want people to be able to get started with a new event without having to have already created artifacts. So, we deliberately don't prompt for a logo image during event creation, since almost nobody with a enw event has one ready. We do mention it in the README: https://github.com/devopsdays/devopsdays-theme/blame/master/exampleSite/utilities/README.md#L17

Once you have created a logo graphic, place it in static/events/yyyy-city/logo.png. (The file MUST be called logo.png.) The sample welcome page has a commented-out element to display a logo named in this way. For front-page use, you also need a square version in static/events/yyyy-city/logo-square.jpg (url configurable in your datafile).

We should probably resolve the png versus jpg thing here just to prevent confusion. (We switched to requiring jpg for the square one - we should either require jpg for both, or not have a requirement for the more free-form logo that is just displayed on the front page.)

[bridgetkromhout]

Bridget to consider name of toggle and to update README. Let's leave logo.png for now but it should get changed to jpg.

[mattstratton]

I am adjusting this to now look for a specific filename. The event logo for the homepage must be in the static/YYYY-CITY directory, and be named logo-square.jpg.

Otherwise it will display the brain logo.

We need to go back through the event data files and remove the unneeded event_logo_square key. It doesn't harm anything being there, other than being confusing.