Prevent users from uploading themes using the default theme names

[poena]

The theme directory blocks users from uploading themes that uses the Twenty YEAR series as their names.

But the function has_reserved_slug only blocks the theme names ranging from Twenty Ten to Twenty Twenty.

Theme names that are already in use are also blocked, so this function only needs to check for future themes.

[Otto42]

In 9384:

Themes: Reserve the next ten years of default theme names. Fixes #4928