* Adds alt attribute with a value of the `mobile-frontend-sitename`
(previously called `mobile-frontend-footer-sitename`). The message was
changed because this affects both the footer and the header's logo.
* Adds a `minerva-footer-logo` class to the logo in footer.mustache in
preparation for changing that element from an h2 to a div. Given caching
issues (old CSS with new HTML; new CSS with old HTML are both
possibilities), we need to do this in two separate deploy cycles.
Bug: T288362
Change-Id: I3ecbf8fdcae72a0ded7f60c559259f312b8ef15d
