Change hardcoded language tag to be read from config file

.Site.Language is actually an object used for multilingual websites. The
correct one to use is .Site.LanguageCode. See
https://gohugo.io/variables/site/#site-variables-list
Also included a default language code, and fixed an HTML syntax error.

This makes it easier to change or override parts of the theme on your
site without losing updates to the theme.

Partial head.html was included in baseof.html, but also in header.html,
causing the body to contain html and head tags.

When using a baseURL such as "/", the extra slash in this partial leads to an invalid href of "//".

Remove extraneous slash from header partial

Extract the summary into its own layout file and use the .Render
function to include the summary, which is the same for all list
templates.

Hugo provides an Author site variable that can be set in the site
configuration. Instead of setting the author variable in the params,
section from this commit on, you have to use the name variable in the
Author section. This adheres more to Hugo standards. See README.MD for
an example.

Page's .Next and .Prev are deprecated and will be removed in a future
release. Replaced these with .NextPage and .PrevPage. Also removed the
links to the previous and next page from the about template. This
assumes there is only ever one about page.

Posts appeared next to each other instead of below each other when the
title and summary were short enough.

See EmielH#9

See EmielH#10

- Cleanup .travis.yml
- Test on multiple versions of Hugo
- Make Travis CI also test branches
- Add HTMLProofer to validate HTML

See EmielH#2

See EmielH#28

See EmielH#30

Added dynamic header menu, Hugo-style
Template from https://gohugo.io/templates/menu-templates
Removed unused i18n

See EmielH#29

Since EmielH#29, these strings are no longer used.

Move menu section to chapter on configuration

* Add code block long lines wrapping

This change is for the code block style - allowing to wrap long lines.

The footer already used .Site.Author.name instead of .Site.Params.Author
and now post info uses this variable as well. You can now safely delete
.Site.Params.Author from your configuration.

See EmielH#36

Tests will now only test the minimal required version for Tale and the
newest version of Hugo.

Fixes EmielH#38

See EmielH#40

See EmielH#51

See EmielH#52 
See EmielH#53

See EmielH#52 
See EmielH#54

See EmielH#56

See EmielH#60

Unlike the other links to favicon, this one was not passed through
relURL.

Co-authored-by: Lukacs T. Berki <[email protected]>

Add table of contents

See EmielH#64

Add dark mode

See EmielH#65

Fixes EmielH#66