Add repost feature (Issue #47)

[NotThatKindOfDrLiz]

This update introduces the ability for users to repost content from one group to any other group they belong to (Issue #47). The feature appears as a "Repost to another group" option on individual posts and allows cross-group sharing while preserving the original content and attribution.

Includes:

• New UI action on posts for reposting
• Group selection modal for eligible destination groups
• Backend support for creating reposted events in selected groups
• This feature improves group-to-group knowledge flow and encourages broader visibility of relevant conversations.

[alexgleason]

This is using kind 6, but I think that's an issue because it would show up in people's home feeds on other clients.

I think perhaps we need to invent a new kind, such as... 666, to mean "repost TO another entity".

[alexgleason]

After changing to kind 666, we need to also pull in kind 666 from whatever filter queries for group posts.

[alexgleason]

We could change this to kind 666, but we'd still need to also do something about the tags (see below)

[alexgleason]

We have an "a" tag targeting the group, and an "r" tag targeting the original group. Not bad... 🤔 Actually maybe this is okay, as long as we change the kind.