Add data files for themes and plugins using setuptools package-data

[dvzrv]

Use setuptools' package-data [1] to unconditionally add all files
below sigal's themes/ dir and all data files below the
plugins/encrypt/static/ dir to a wheel.
This works without any SCM integration and without MANIFEST.in.

Fixes #500

[1] https://setuptools.pypa.io/en/latest/userguide/datafiles.html#package-data

[codecov]

Codecov Report

Merging #503 (ebedd9f) into main (6bd9cd3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #503   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files          25       25           
  Lines        2062     2062           
=======================================
  Hits         1829     1829           
  Misses        233      233           

[saimn]

Thanks for the PR!
With include-package-data = true this shouldn't be needed, at least from what I understand from https://setuptools.pypa.io/en/latest/userguide/datafiles.html#subdirectory-for-data-files, but package data has always been a confusing thing with setuptools so fine by me :)