[fix bug 1009570] Improve a11y of SVG and img alt text content on /firefox/desktop

* Added `<title>` text to inline SVG document images (I figured `<desc>` was probably overkill for these images).
* Added `alt` text to a couple of img's on `desktop/customize` where I think appropriate (most images are largely presentational I think).
* Changed image links in the Themes section to use real `<button>` elements.

There are some new strings introduced in this change, but I think we should review first and decide that a.) the new `alt` text is appropriate and b.) there's nowhere else in these pages where we should also be adding alternate text.