Convert discussion block's html from mako template into django template format

[salman2013]

Description

In this PR i converted discussion's html files from mako into django template.

I converted following files. _discussion_inline.html‎ _discussion_inline_studio.html‎ _filter_dropdown.html _thread_list_template.html _underscore_templates.html‎ discussion_board_fragment.html‎

Ticket Reference: https://github.com/orgs/openedx/projects/55/views/1?pane=issue&itemId=106903650&issue=openedx%7Cedx-platform%7C36539

Acceptance criteria

• html template should be refactored to django template with minimal code changes
• Discussion XBlock experience shouldn't be changed
• All test cases should be passed on the edx-platform

How to test this PR

To enable discussion xblock:

tutor plugins install forum
tutor plugins enable forum
tutor dev do init -l forum

To show the builtin discussion xblock component on unit page:

• Go to studio admin site.
• Go into discussion configurations (/admin/discussions/discussionsconfiguration/)
• Update the provide to 'legacy'

To show the legacy discussion view from discussion tab:

• Go to studio admin site.
• Go into flags (/admin/waffle/flag/)
• disable this flag 'discussions.enable_discussions_mfe'

Testing results

Sanbox testing results.