Add admin key to the "invite people" sharing text input

[nikolaiwarner]

Like the cli, the admin key should be added to the cabal key so that moderation is set up for the person you share with.

[cblgh]

@nikolaiwarner this is how it's done in cli atm https://github.com/cabal-club/cabal-cli/blob/master/util.js#L15-L23 with

state.moderationKeys = cabal.core.adminKeys.concat(cabal.core.modKeys)

[wittjosiah]

@nikolaiwarner @cblgh I gave this a shot here master...wittjosiah:feature/share-mod-key, happy to submit a PR if that's helpful!