Certain tests involving HTTP requests were sometimes hanging
indefinitely, so this uses Mockito for HTTP mocking. This seemingly
resolves the issue and makes these tests more reliable.

The existing can_fail_404_links test has been renamed to
can_fail_unresolved_links, to represent what actually occurs in the
test. The can_fail_404_links test now deals with a proper 404
response.

Just to be clear, the check_site test in the site component will
still create outgoing HTTP requests (due to the URLs used in the
test_site), so this commit only uses HTTP mocking where possible.

* maybe_slugify() only does simple sanitation if config.slugify is false

* slugify is disabled by default, turn on for backwards-compatibility

* First docs changes for optional slugification

* Remove # from slugs but not &

* Add/fix tests for utf8 slugs

* Fix test sites for i18n slugs

* fix templates tests for i18n slugs

* Rename slugify setting to slugify_paths

* Default slugify_paths

* Update documentation for slugify_paths

* quasi_slugify removes ?, /, # and newlines

* Remove forbidden NTFS chars in quasi_slugify()

* Slugification forbidden chars can be configured

* Remove trailing dot/space in quasi_slugify

* Fix NTFS path sanitation

* Revert configurable slugification charset

* Remove \r for windows newlines and \t tabulations in quasi_slugify()

* Update docs for output paths

* Replace slugify with slugify_paths

* Fix test

* Default to not slugifying

* Move slugs utils to utils crate

* Use slugify_paths for anchors as well

* cargo fmt & clippy

* Skip anchor checking for URL with prefix in config

Add method get_translation(lang, key) into Config struct that retrieves
translated term from parsed configuration or error when either
desired language or key is missing.

Use the new method in Trans struct implementing global Tera function
trans().

Add unit test to cover both happy and error path for translation
retrieval in both config and templates crate.

* Set up CI with Azure Pipelines

[skip ci]

* Remove other CI + remove newline in anchor link tpl

* Some more Windows specific changes

Close #686

Closes #607

Closes #576

Fix #546