Basic test coverage round 2

Problem/Motivation

Some basic functionality is missing test coverage.

Proposed resolution

Create tests for

• unsubscribing aka disabling
• tokens
• user preferences
• user/node/comment deletion
• mail body
• extra field display
• settings form

I don't think we want to create all these tests at once, it's more helpful to commit them in parts. I think unsubscribing, tokens and user preferences are top priority, because they are basic non-admin features, and the rest can be done in separate issues?

Remaining tasks

User interface changes

API changes

Data model changes