"I'm Going" checkbox should expire after event date has passed

[woznaut]

currently, users are able to select "I'm going" after an event date has passed. the mobile UI should either show this control as disabled or hide it completely. I think hidden is preferable assuming the level of effort is the same.

• Pull from upstream to get the latest code
• Checkout the feature branch
• Compile and run the code
• Develop the feature
• Test locally on a device
• Create Pull Request