Fix alt attribute in img tags

[codekendo]

app/templates/base.html

img tag needs an alt attribute.

https://moz.com/learn/seo/alt-text

[thyrgle]

Hey man, thanks for all of these! Definitely let me buy you some coffee or tea some time.

Note to self: Currently alt has been added to base, but there are other alts needed, leaving this open until I fix them all. Probably need to add some integration test...