chore(convert): improve HTML Jinja2 template

[jeertmans]

As suggested by @yunusey in #442.

TODO:

• Fix weird indentation if conditional slide attributes
• Wait before fix(convert): Whitespace issue in default RevealJS template #442 is merged
• Update HTML example in docs to reflect changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.30%. Comparing base (c0a240d) to head (f3d8065).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   79.30%   79.30%           
=======================================
  Files          22       22           
  Lines        1822     1822           
=======================================
  Hits         1445     1445           
  Misses        377      377           

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