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 changes the permission check so it uses the policy on Noteable
instead of Project. This prevents bypassing of rules defined in
Noteable for locked discussions and confidential issues.

Also rechecks permissions when reply_to_discussion_id is provided since the
discussion_id may be from a different noteable.

This changes the permission check so it uses the policy on Noteable
instead of Project. This prevents bypassing of rules defined in
Noteable for locked discussions and confidential issues.

Also rechecks permissions when reply_to_discussion_id is provided since the
discussion_id may be from a different noteable.

Enable frozen string in:

* app/presenters
* app/policies

Partially addresses #47424.

This prevents performing the requests, and disables all emoji reaction buttons

Adding the necessary API for the new /snippets Restful resource
added with this commit. Added a new Grape class `Snippets`, as
well as a `PersonalSnippet` entity.

Issue: #20042
Merge-Request: !6373
Signed-off-by: Guyzmo <guyzmo+gitlab+pub@m0g.net>