[5.1] tinymce hardening fix

[brianteeman]

This addresses two issues resolved in tinymce 7 that we can backport to 6.8

I checked with @joomla/security before posting

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[SniperSister]

I have tested this item ✅ successfully on 5092ce4

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[Quy]

I have tested this item ✅ successfully on 5092ce4

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[Quy]

RTC

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[brianteeman]

I made this pr for 5.1 on the assumption that there will not be anothher 5.0 release. If that is incorrect then it should be backported to 5.0

[bembelimen]

Thx

[N6REJ]

just an fyi, this breaks video's that are embedded with iframes!
https://forum.joomla.org/viewtopic.php?f=834&t=1008580