Add sponsors to OpenCon Cam event website

[KirstieJane]

• [x] Publons
• [x] Overleaf
• [x] Wellcome Trust
• [x] Copernicus
• [x] PeerJ
• [x] eLife
• [x] CrossRef
• [x] SynBio
• [x] Cambridge Open
• [x] Office of Scholarly Comms
• [x] PLoS

[KirstieJane]

Publons logo doesn't have a background - need to fix