New plain-text editor, updated third-party packages in backend and more

Today's update (b827b0037) brings with it the following changes:

Performance improvements

Upgrade to newest version of Perl (5.24.1) and CPAN modules to get an overall performance improvement.
The main dropdown menu (top left) is written in a slightly faster way which should make all normal page loads slightly faster.

When the WYSIWYG editor (TinyMCE) is disabled, which is common for power users, you now get a plain-text editor called CodeMirror which has better editing features than a normal HTML textarea. It has tab indenting, syntax highlighting and code folding, to name a few. It is also possible to customize how you want the editor to function on a site-by-site or user-by-user basis.

Perlbrew and Pinto are now used to manage Perl and CPAN packages. This ensures we always used the same version of perl and CPAN modules on all installations. This should avoid test failures and other unexpected behavior because different versions of third-party dependencies are used. As this changes a lot of the underlying foundation, there is a higher chance of regressions from this update. Get in touch with support if you notice something out of the ordinary that our testing didn't catch.
The type of objects which supported WYSIWYG mode and file upload was updated. This should allow e.g. text files to be uploaded to overwrite existing content (you don't have to paste the file into the editor now).
The first tab in object editor is activated when object type is changed to allow editor to initialize with visible viewport, avoiding potential issues.
Moved some inline styles in request_add_edit template over to the standard stylesheet, simplifying the template.

The default width of 12.5em on textareas from normalize.css was reset, because it conflicts with our usage of rows/cols attributes to set the size of the editor.
Dropdown boxes rendered using element_select without onchange argument are no longer rendered with an empty onchange attribute, allowing event handlers to be applied normally.
More modern JavaScript code is now used to show/hide the language metadata dropdown menus in object editor.
Fixed some JavaScript errors related to TinyMCE when changing type in the object editor.
Non-ASCII text was sometimes encoded in a wrong way in email headers and when downloading files. This should now be fixed.

The transcode checkbox in the object editor was removed because the video transcoding feature is no longer supported.