Add activity request template

[totally-not-frito-lays]

Is your feature request related to a problem? Please describe.
Currently new activity ideas are set to be stored in a mega thread issue. This isn't very scalable, especially considering we have many new volunteers in the onboarding process currently.

Describe the solution you'd like
We should add an activity request template so that new activity ideas are stored as individual issues instead of being in a mega thread.

Describe alternatives you've considered

1. keep the mega thread
2. [proposed solution] create new issues for new activity ideas

Additional context
Add any other context or screenshots about the feature request here.