i18n updates

Updated cs_CZ, de_DE, fr_FR & tr_TK
Fixed wrong use of `__FILE__` 

Using `__FILE__` in a file in a subdirectory results in a wrong path for the plugin url, using `WPSEO_FILE` fixes the issue.
Remove unneeded `dirname` calls.
Prevent regex from being too greedy.
Moved constant WPSEO_FILE to main plugin file

Also replaced __FILE__ with WPSEO_FILE in wp-seo-main.php
Merge branch 'master' into release/1.5
Removed PHP version check, WP requires 5.2.4 or higher
Moved Activation and deactivation hook to wp-seo-main.php and the hooks are now using WPSEO_FILE

Premium also needs these hooks to run.
Moved WPSEO_VERSION to wp-seo-main.php

We also need this constant in Premium
Result of quick check of the @todo's. Removed check for taxonomy_meta option in reset method as it is not needed. (method uses `get_option_names()` which checks the `$include_in_all` property which for taxonomy_meta = false).