Fixes composer dependencies: Package phpunit/phpunit-mock-objects is abandoned
We cannot support v8 yet because of errors like:
Declaration of MailFunc::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void
It would require dropping PHP < 7.1 support.
Now instead of <pre> we use <div class="pre"> styled with monospace
font. We replace whitespace characters with non-breaking spaces where
needed. I.e. plain text is always unwrappable, until it uses format=flowed,
in such a case only flowed paragraphs are wrappable.
Also conversion of text to HTML in compose editor was modified in the same way.
Aleksander Machniak