Decide about how conversion site matching works

[martinthomson]

#139 defines an impression record that includes a set of conversion sites. This is probably a little unusual, in that many ads will only target actions on a single site (noting that site entails multiple origins).

There are two related questions to resolve related to how conversion site information is saved for impressions.

(1) Empty conversion sites set

Should it be possible to save an impression with an empty set of conversion sites?

The change in #139 allows that and says that any site can find and use that impression in its conversion logic.

The alternative is to force this set to be non-empty (as #56 does). What will decide this is whether it makes sense for impressions to be converted on any site, or if it is possible for an impression to be saved without knowing a priori where it might convert. Neither of these seem likely to me.

(2) Singular conversion site

It's definitely true that some ads could convert on multiple different sites, but if the set of conversion sites cannot be empty, we have another choice: we could decide that it is simpler overall to have the conversion site be a simple value, not a set.

The consequence of this is that impressions that might convert at multiple different locations would need to be saved once for each conversion site. If most ads only ever convert in one place, or even a very small number of places, that would result in a design that is simpler to implement and explain.