Fix activity heatmap to show proper day name.

What does this MR do?

This MR fixes issue #19909 (closed).

Are there points in the code the reviewer needs to double check?

Not necessarily, it's pretty straightforward, buuuut

There was some discussion about whether GL wanted the activity heatmap to start on Monday or Sunday, this fix makes it start on Sunday, if GL is wanting it to start on Monday then some more substantial changes need to be made on the calendar.js.coffee script.

Why was this MR needed?

The amount of day names that were off from their dates was too damn high! :P

What are the relevant issue numbers?

Directly #19909 (closed)

Also some related discussion (feature discussion): #19282 (closed), #5988 (closed)

Screenshots (if relevant)

Currently on GL.com

After MR

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together