Alt tags for accessibility

This was never merged! It got merged into new-splash-pages, but that branch was never merged into dev or master.

See individual commit messages.
Sorted all SVGs and company logos across the site.
Still need to sort out alt-text for some of the larger images, such as the payment timetable and software screenshots.

This HAS been checked with a screenreader (Voiceover on iOS) and everything that's been added in this PR sounds great.
If you're trying to better describe the image, perhaps "GoCardless Dashboard interface" might be better? Just wondering if "GoCardless Web Dashboard" sounds a bit weird and sounds more like another product than an explanation of the image.