Remove legacy Jinja2 templates

[Valkyriezz]

Summary

• Delete all 21 Jinja2 template files from tahrir/templates/
.html template files (index.html, user.html, badge.html, badge_full.html, leaderboard.html, explore.html, explore_badges.html, report.html, about.html, diff.html, admin.html, builder.html, thingiview.html, assertion_widget.html, tags.html, error.html, _base.html, _badge-base.html, _functions.html,test.html)
• These templates are part of the legacy frontend being replaced by React
• No API endpoints or React frontend code references these templates

Fixes part of #904 (Task 1)

[gridhead]

@Valkyriezz, please also include the removal of the views that require the usage of the templates that you just happened to have removed, because the application will not initialise if it does not end up finding the templates that it expects to find at the moment. Also, please sign the commits, if you have not done that already before another round of reviews.

[Valkyriezz]

Hi @gridhead, I've addressed your feedback. Along with removing the 21 Jinja2 template files, I've now also removed the 15 view functions that depended on them.
The JSON/API endpoints, RSS feeds, and redirect handlers are all preserved only the render_template() code paths were removed. Error handlers have been converted to return JSON instead of rendering templates.
Since this is a larger change touching 8 view files, I'd appreciate your review to make sure nothing was missed.

[gridhead]

@Valkyriezz, you still do not seem to have signed your commits after having asked for it multiple times. When you do that, please also ensure to squash all your commits into one.

[Valkyriezz]

I think removing the views is part of Task 2 and already has a PR, but feel free to review mine as well :) will make changes accordingly