Add used fields to Calendar Time Event in NIP-52

[vicariousdrama]

Cornychat.com relies on a t tag of audiospace for filtering. Flockstr.com relies on optional and previously undocumented audio and image tags for rendering.

[staab]

Using t for metadata not meant to be read by users is a mistake. Imagine a valley girl shouting "hashtaaag #audiospace!" as a litmus test. I would prefer corny chat switch to using NIP 32 labels if possible.

[vicariousdrama]

It's early enough that I can change the implementation in Corny Chat for this to be able to identify audiospace events.

If t is exclusively for hashtags, then I need to change some others in use as well

For context, Corny Chat is also adding a t tag for cornychat.com which could probably be an l tag instead. In addition, a deleted tag is being added which I may be able to refactor into using expires

[vicariousdrama]

Kind of a pain to sort out what tag should be used as looking at NIP32, labels are a kind 1985 event, and I need to add this meta information to kind 31923

[staab]

See "self-reporting" in NIP 32

[vicariousdrama]

Switched to label tags and namespace in place of hashtag tags

[tyiu]

I think we should push this PR through after replacing the about tag with summary, as discussed above. After that, we can merge the PR. @vicariousdrama