Replace post lock icon with a dashicon

[melchoyce]

The post lock icon is still using old, pre-3.8 styles:

Let's replace it with .dashicons-lock .

[nacin]

Dropping a comment on this ticket as the new ticket notification accidentally did not go out.

[melchoyce]

26796.diff​​ removes the lock icon from wp-admin.css and replaces it with the lock dashicon, then makes it a lighter color in colors.css.

[kirasong]

Refreshed after CSS splitting funtimes

[kirasong]

Refreshed after wp-admin.css and colors.css splitting/merging.
The patch​ applies to src/.

Two things:

• Unsure if edit.css is the right place for the color rule. Seems like it may belong in list-tables.css(?), since the icon isn't used in the edit screen, but I put it there because it matched up with previous patch placement pre-split.
• Removed the@print section from list-tables.css because the patch removes the last rule. If this was incorrect, or it should be left for organization's sake, it can certainly be added back.

[nacin]

• We can consolidate this in list-tables.css.
• Agreed with removing @media rule.

[nacin]

In 27392:

Convert the post lock icon to a dashicon.

props melchoyce, DH-Shredder.
fixes #26796.