Handle rewrite for "alias"

Modify the static methods of the mapping interface to be handling rewrites for aliases.

Write tests regarding these rewrites in `AbstractRecipientRewriteTableTest`:

```
Given an alias mapping from 'bob-alias@domain.tld' to 'bob@domain.tld'
When I call getMappings from 'bob-alias@domain.tld'
Then a Mappings containing only 'bob@domain.tld' is returned
```