It persist temporary personal snippets under
user/:id namespaces temporarily while creating
a upload record to track it. If an user gets removed
while it's still a tmp upload, it also gets removed.
If the tmp upload is sent, the upload gets moved to
personal_snippets/:id as before. The upload record
also gets updated to the new model type as well.

String#underscore isn't particularly slow, but it's possible for us to
call it many times in a users autocomplete request, with mostly-static
values ('User', 'Group', etc.). We can memoise this and save a
surprising amount of time (around 10% of the total request time in some
cases).

This is now entirely handled by `create_note`:

1. Project snippets prevent `create_note`.
2. Uploads already only support routing for personal snippets.

This simplifies some policies and access checks, too!

This style change enforces `return if ...` instead of
`return nil if ...` to save maintainers a few minor review points

Also removed unnecessary comment

Enables frozen string for the following:

* app/controllers/*.rb
* app/controllers/admin/**/*.rb
* app/controllers/boards/**/*.rb
* app/controllers/ci/**/*.rb
* app/controllers/concerns/**/*.rb

Partially addresses #47424.

An upcoming update to rubocop-gitlab-security added additional
violations.