Use notice for context when deciding who @nickname refers to

Use notice for context when deciding who @nickname refers to

In a federated system, "@nickname" is insufficient to uniquely
identify a user. However, it's a very convenient idiom. We need to
guess from context who 'nickname' refers to.

Previously, we were using the sender's profile (or what we knew about
them) as the only context. So, we assumed that they'd be mentioning to
someone they followed, or someone who followed them, or someone on
their own server.

Now, we include the notice information for context. We check to see if
the notice is a reply to another notice, and if the author of the
original notice has the nickname 'nickname', then the mention is
probably for them. Alternately, if the original notice mentions someone
with nickname 'nickname', then this notice is probably referring to
_them_.

Doing this kind of context sleuthing means we have to render the
content very late in the notice-saving process.