The link service shouldn't tell a user about a link they themselves shared before

[allenhumphreys]

The link service is about coalescing conversations around recent topics. Documentation links are often shared repeatedly, and often shared by the same user who may be a subject matter expert over and over again. Generally, we think the users are aware they're sharing these kinds of links multiple times and it doesn't go against the general conversation goals.

So, we should update the link service to not chime in if the historical link reference is by the same user posting the current link.

note: This may be less of an issue once #26 is implemented. Though, it'll still be relevant but it could affect prioritization.