Switch contribution log to OpenCollective page

[jc00ke]

Hello!

@joelgaff and I have set up an OpenCollective account to better serve the community. This will take the place of the manual reimbursement collection I've handled for years.

I propose the following updates:

• On home page, make participating via contributions more prominent
• On contribution log, add notice that it's been deprecated and point to OC

I would prefer the contribution log to have a $0 balance, so once it is so via OC I will update the log and then it can be fully deprecated.

Thoughts? Ideas?

[joelgaff]

Seems reasonable to me. Thanks for helping get OC set up, Jesse!