Fix EZP-21986: Removing an underline removes the surrounding custom tag

JIRA: https://jira.ez.no/browse/EZP-21986

# Description

In Online Editor, if both an inline custom tag and the underline tag are applied to the same word, removing the underline with the OE toolbar will also remove the other custom tag.

This patch changes the way OE generates the underlined text in TinyMCE to use the `<u>` tag and makes sure the server side is consistent with this change.

# Tests

Manual tests + unit tests for the server side part