The goal of this PR is to make Following more intuitive.
### Old Behavior
Previously, following was scoped to a project. In order to follow
someone in a given window, the window needed to contain a shared
project, and the leader needed to be present in the project. Otherwise,
following failed.
### New Behavior
* You can always follow **any** participant in the current call, in any
pane of any window.
* When following someone in a project that you're both collaborating in,
it works the same as before.
* When following someone in an unshared project, or a project that they
don't have open, you'll only get updates about the leader's views that
don't belong to a project, such as channel notes views. When the leader
focuses a file in a different project, you'll get the "follow $LEADER to
their active project" indicator
### Todo
* [x] Change db schema and RPC protocol so a project id isn't required
for following
* [x] Change client to allow following into non-project items regardless
of the leader's project
* [x] Assign colors to users in a way that doesn't require users to be
in a shared project.
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