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.