Fix default expiry date to be the 25th

Done through the GitHub UI as I wanted to get this in quick
Merge branch 'voting-button-fix' into production
Add Gibraltar logo to the project.

Image file came from Gibraltar as-is: I've not tweaked it at all.
Add Gibraltar logo to Sponsors page.
Add Gibraltar logo to side bar.
Update with transparentised image.

Replace the logo provided by Kendall with the tweaked one from
@adrianbanks.