calibre: will crash if markdown is configured in custom column

[bennyandresen]

Describe the bug
calibre offers the ability to create custom columns and one of the options is:
Interpret this column as: Plain text formatted using markdown

If this option is used, the current version of calibre will crash as it lacks python-markdown in its environment.

libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
Traceback (most recent call last):
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/gui2/book_details.py", line 896, in show_data
    self.book_info.show_data(data)
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/gui2/book_details.py", line 638, in show_data
    html = render_html(mi, css(), self.vertical, self.parent())
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/gui2/book_details.py", line 110, in render_html
    use_roman_numbers=config['use_roman_numerals_for_series_number'], pref_name=pref_name)
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/gui2/book_details.py", line 192, in render_data
    rating_font=rating_font(), default_author_link=default_author_link())
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/ebooks/metadata/book/render.py", line 126, in mi_to_html
    val = markdown(val)
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/library/comments.py", line 138, in markdown
    from calibre.ebooks.markdown import Markdown
  File "/nix/store/wzyg72rsmxblk4wazrw6ryl3hng7p5kl-calibre-3.48.0/lib/calibre/calibre/startup.py", line 69, in load_module
    return import_module(fullname[len('calibre.ebooks.'):])
  File "/nix/store/bzf8x5kpv3x4qi09xq2grnvccakvy7gz-python-2.7.17/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
ImportError: No module named markdown
[1]    15851 abort (core dumped)  calibre

To Reproduce
Steps to reproduce the behavior:

1. Have custom formatted column configured in calibre
2. start calibre
3. crash, see above Traceback

Expected behavior
No crash, including working markdown formatting in the custom column

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 4.19.81, NixOS, 20.03pre199897.471869c9185 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(benny): `""`
 - channels(root): `"nixos-20.03pre199897.471869c9185"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:
@domenkozar @pSub @AndersonTorres