This commit adds support for the `in:<topic notification level>` query
filter. As an example, `in:tracking` will filter for topics that the
user is watching. Filtering for multiple topic notification levels can
be done by comma separating the topic notification level keys. For
example, `in:muted,tracking` or `in:muted,tracking,watching`.
Alternatively, the user can also compose multiple filters with `in:muted
in:tracking` which translates to the same behaviour as
`in:muted,tracking`.