To be able to implement OE features listed in EZP-28030, striked <s> text needs to be supported.

Currently only <del> is supported by RichText, but this means deleted text and has a different contextual meaning.

After a bit searching it seems we should do the following mapping for these tags so we can also continue to support <del> which might be useful in the future.

• <s> => <emphasis role="strikedthrough">
• <del> => <emphasis role="strikedthrough" revisionflag="deleted">

This also means that the meaning of strikedthrough text already migrated from eZ Publish will now correctly be displayed as strike in the editor and view, instead of being converted to <del> which editor did not support and which has other meaning.