Enforce tab indentation in templates (#15289)

Enforce tab indentation in templates (#15289)

* Enforce tab indendation in templates

This adds editorconfig-checker [1] to lint the template files so they
conform the editorconfig files. I fixed all current identation issues
using the fix mode of eclint [2] and some manual corrections.

We can extend this linting to other files later, for now I'd like this
PR to focus on HTML template files only.

[1] https://github.com/editorconfig-checker/editorconfig-checker
[2] https://github.com/jedmao/eclint

* fix indendation

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>