WYSIWYG editor allows setting underline, but this is stripped out by Bleach

One-sentence description

In the admin, the WYSIWYG editor allows setting underline, but this is stripped out by Bleach

Current behaviour

No underline

Expected behaviour

underline

Steps to reproduce

Create e.g. an announcement
Use the U button
Save
See that there's no text-decoration: underline in the output.

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