FIX: Boosts in reviewables should link to posts rather than topics

[small-lovely-cat]

There exists a bug in the reviewables, when a boost is reported, the link of it is pointed to the topic rather than the specific post, which makes finding the boost difficult when there are large number of posts in the topic.

This commit add target_url so that the link is pointed to the specific post of the boost rather than the topic.

Before:

After:

Also added some spec to it.

FIX: Boosts in reviewables should link to posts rather than topics (#9)

[nat (Natalie T)]

Thanks for your contribution again, our teammate @zogstrip has merged your PR