Reduce duplicated code on visits components and reducer

[acelaya]

There's currently a few sets of components and reducers to handle visits:

• Short URL visits
• Tag visits
• Orphan visits
• Non-orphan visits
• Domain visits (TBD)

All of them have a relatively close handling, as the UI has only small variations.

Currently there's a couple of helper functions used to reduce code duplication, but there's still room for improvement.

Try to find a way to reduce duplicated code (specially on tests), without making it even more complex to understand.

[acelaya]

As part of #7 and #9, there have been many refactors, so I'm closing this as completed