Some proposed improvements to user invites

[techAPJ (Arpit Jalan)]

@codinghorror

The invite button still does not appear for private topics, but should, only if you are an admin.

Implemented:

FEATURE: show Invite button to admin for private topics (#2506)

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@codinghorror

Right now there is zero topic context included in a topic invite… but there should be.

Implemented:

FEATURE: include topic context in topic invite (#2521)