[Event Settings] Fix card grant tab not showing

[Luke-Oldenburg]

Summary of the problem

Describe your changes

[garyhtou]

Instead of doing an after_update which creates a CardGrant for all organizations, let's always show the Card Grant settings tab if the event plan allows for it. On that tab, if the card grant setting doesn't exist, then show an "Enable card grants" button.

[garyhtou]

The downside of after_update is that we'd have a bunch of unused card grant settings. When we need to change the default settings, we'd then need to determine which are actually used vs unused, communicate breaking changes, and roll out changes to these records.