EZP-22509: HTML pasted into literal ends up outside literal tag when the source is not plaintext

JIRA: https://jira.ez.no/browse/EZP-22509 (follow up of https://jira.ez.no/browse/EZP-22313 fixed in #905)

# Description

When trying to paste some code that comes from a non plain text source in a literal area, the literal is removed or the code is pasted outside of it. This happens because of the default setting of the paste plugin that tells TinyMCE to create some paragraphs when pasting as text...
So the fix in 9bfa39c changes this setting so that no paragraph is added. While at it, I also fixed a display issue in 2ae61f6 where long lines are not wrapped into the literal area and improved the conversion to plain for Firefox in 8196cd2 so that it keeps the line feed when the code comes from a pre tag.

# Tests

manual tests