Mail: include markdown in mails

[lukastocker]

• add markdown rendering. Also always escape HTML in markdown output.
• use UI form in ilMailFormGUI and add Markdown for Message.
• enable markdown for free text mails.
• adjust PHPUnit Tests
• include new functions in form and buttons to avoid a redirect of the page if the user clicked onto the placeholders to insert them into the mail.

Notes regarding commits in this PR:

Mail: decode HTML in mail body of free text mails else ... => This commit is optional. If needed I can remove this one. Mail: Replaces all known HTML tags with... => This commit is optional too, but I would recommend to include the commit to this PR.

Inn addtion I created two more PRs you could check out:

1. Underline Extension for Markdown
2. Adjusting the Markdown's Headline configuration, which is also needed for the underline extension PR