Add pagedown javascript resources
Remove Pagedown Markdown Editor

* pagedown editor is too rigid, we won't use it but only the converter
  and the sanitizer
Firt implementation of a two pane preview for status editor

* using bootstrap tabs
First working implementation of a two-tab editor with preview

* preview on second tab
* auto-hide/show of tabs
* use pagedown converter and sanitizer
Add i18n support to editor tabs labels
Hide update status button on page show

* the button is only displayed when the form is opened
Markdown preview for reply editor
Replace Pagedown with marked for markdown preview

* marked is already used for status display
* allow correct handling of special formatting rules (user name with
  link for example)
Merge pull request #275 from PierreRust/markdownPreview

Markdown preview