Remove extra css class markdown-area which prevented attachments upload.
Warn people about not exposing at a time they can still do something about it.
Prevent 500 after merge MR if you check remove source branch

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'fix-500-after-merge' into 'master'

Fix 500 on MR diff page after merge and branch was removed

Fixes #1847

See merge request !1345
Release manager should doublecheck the everyone has been mentioned in the blog post.
Merge branch 'double-check-blog-post-mention' into 'master'

Double check blog post mention

See merge request !1347
Merge branch 'note-env-variables-in-installation' into 'master'

Note env variables in installation

See merge request !1342
Merge branch 'remove_extra_css_class_from_markdown' into 'master'

Remove extra css class from markdown

Extra css class prevented attachments uploader from functioning correctly.

This caused dropzone wrapper classes to be appended to the wrong class and further prevented the dropzone gem from functioning correctly.

Removing this extra class allows attachment upload to function again while keeping the feature introduced here intact.

Fixes #1831

See merge request !1338