Missing sky implications

BUR #15695 has been approved by @nonamethanks.

create implication purple_sky -> sky
create implication grey_sky -> sky
create implication black_sky -> sky
create implication yellow_sky -> sky
create implication pink_sky -> sky
create implication white_sky -> sky

Since it looks like the usage restrictions of sky have been decided to apply to colored_sky tags as well.

BUR #15696 has been approved by @nonamethanks.

create implication overcast -> sky

Adding this one separately since I'm not 100% sure if it should apply when the sky is technically covered.

Other color tags already imply sky, so meh.

In theory you're not supposed to use most of them unless the sky is really visible, but yeah, good luck enforcing that.

The bulk update request #15695 (forum #235775) has been approved by @nonamethanks.

The bulk update request #15696 (forum #235776) has been approved by @nonamethanks.

BUR #18662 has been rejected.

create implication aqua_sky -> sky

A missing colour implication. It does visibly exist IRL.

This bulk update request is pending automatic rejection in 5 days.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #18662 (forum #248995) has been rejected by @DanbooruBot.

Moved manually to blue sky or green sky.

BUR #27036 has been approved by @nonamethanks.

create implication green_sky -> sky

Missed this one.

The bulk update request #27036 (forum #283725) has been approved by @nonamethanks.