Add the ability to assign custom tags to submissions

[nathandyer]

Description

By adding the ability to set custom tags on submissions, it makes managing a large number of submissions easier, especially in newsrooms where there are many journalists reviewing submissions and interacting with the Journalist Interface.

This would also potentially be useful for implementing features on the development side (for example, "tagging" submissions that are queued for deletion, in an effort towards implementing #7094)

How will this impact SecureDrop users?

This will make it easier to manage submissions within a newsroom.

How would this affect SecureDrop's threat model?

These additional tags would increase the metadata about a submission, which might have a negative impact in the event that a server is compromised. It also adds the potential for misuse, where a tag could be set which reveals sensitive information about the submission itself.

User Stories

As a journalist, I want to be able to tag submissions so that my colleagues can know more about the status of a submission, and so that we can better manage them as a group.