Make autocomplete complete end to block tags

For example, tab complete on {% comment %} should also fill out {% endcomment %} and move the cursor in the middle.

Start by locating the autocomplete implementation that handles Django template block tags, using {% comment %} and {% endcomment %} as the concrete case. Confirm that completing an opening block inserts its matching closing tag and places the cursor between them; verify the behavior with the project's existing checks or manual editor testing.

Written by the indexing model from the issue text.