add alt text to sponsor logos

[altsalt]

There doesn't currently seem to be a way to add a hover-over description. For now this doesn't really need to be visually apparent, but should add to the image alt-text or something.

[AndrewKvalheim]

They already have alt text:

seagl.github.io/_includes/sponsor_wall.html

Lines 15 to 17 in 89b61b4

	<img src="{{ sponsor.logo.horizontal }}" alt="{{ sponsor.name }}">
	{%- elsif sponsor.logo.square -%}
	<img src="{{ sponsor.logo.square }}" alt="{{ sponsor.name }}">

You mean they should have a title or caption too?

[altsalt]

Hmm, yeah it is tricky, I suppose I want title functionality with a caption due to not misusing the title tag. The idea would be that the sponsor sends us a brief description which might be displayed on hover, while we may eventually display a longer description as a modal or something when clicked.