Improve theme directory translation strings

[ramiy]

Ok, the patch has 3 changes:

1. I added translator comments. /* translators: context */
2. I've moved the html <code> tags from the translation strings using sprintf().
3. And I updated few strings with the same context.

[ramiy]

Source File: https://meta.trac.wordpress.org/browser/sites/trunk/wordpress.org/public_html/wp-content/plugins/theme-directory/class-wporg-themes-upload.php

[SergeyBiryukov]

#1244 was marked as a duplicate.

[ocean90]

In 1910:

Theme Directory: Improve strings with style.css references and add translator comments.

Props ramiy,SergeyBiryukov.
Fixes #1058.