docs, update: tell users to copy entire `data` dir

Previously, the document told users to only copy `data/db.sqlite`.
However, if there are attachments uploaded (stored in `data/files`),
they will be lost.
To solve this, this commit changes the doc so it tells users
to copy the entire `data` dir.
Fix date due ordering on the dashboard, see #403
Description should not be mandatory, see #400
Move and clean some templates to a subfolder
Add Markdown preview for textarea, see #407