Prevent accidental addition of users to talentpool

Bug: When asking for a review for a user that isn't currently nominated
using the `!talentpool get_review <userid>` command, the user is added
to the talentpool `watched_users` cache, causing their messages to be
relayed to the talentpool watch channel.

Steps to reproduce: Use `!talentpool get_review <user_id>`, where
`<user_id>` is the ID of a user not currently nominated. The command
will correctly reply that the user isn't nominated, but their ID will be
added as a key to the defaultdict nonetheless.

Solution: replace all regular getitem usages with `.get(<user_id>)`, as
the Reviewer should never insert IDs using the regular defaultdict path.

Additional note: I've replaced all occurrences of regular getitem access
into the defaultdict, even those that are normally not reachable with
the id of a user that's currently not nominated, to prevent a future
refactor from accidentally introducing this bug again.

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