Drop "logo" from logo image alt text

[jonathanhefner]

From https://www.w3.org/WAI/tutorials/images/tips/:

Usually, there’s no need to include words like “image”, “icon”, or
“picture” in the alt text. People who can see will know this already,
and screen readers announce the presence of an image.

And from https://www.a11yproject.com/posts/alt-text/:

Make sure the description of the image is useful. For example, if the
image is of a company's logo, the alt should be the company's name.
The word "logo" is not necessary or useful as part of the alternative
text.

Additionally, this commit uses project_name in the alt text instead of hardcoding "Ruby on Rails".