feat: YEC imagery to be added between subnav and donate form

As per #78, we must ensure that there's a spot for a big YEC callout between the subnav and the donate form. The particular callout this year consists of three illustrations; as the pagewidth shrinks below the point where all three could be displayed at full size, we should hide the two flanking the center illo.

This commit implements this feature as requested. A few additional items have been added in support of this feature:

• Boilerplate alt text has been provided for the images (and please tell me what you'd like it changed to!)
• A variable, is_yec, has been added to settings.py, which now feeds the CiviCRM repository's is_yec internal variable and is_yec() method. settings.is_yec acts as a flag which is read by the donate form template, and based on its value, it hides or shows content found in yec.html.jinja. For the purposes of reviewing and testing the site, it is currently set to True, but should probably be set to False before we launch (as the YEC is set to begin about a month after sitelaunch).

Closes #78.