Improve month and year selects

[airblade]

In order to look a little cleaner, the month and year <select>s have appearance: none and a custom down-arrow.

However there are a few problems:

• For months with shorter names, the arrow is far away from the name.
• The arrow is not clickable.
• The focus ring encompasses the name only, not the arrow, and doesn't have enough padding.

[airblade]

For the arrow being far away from the name:

• We could abbreviate all month names to 3 letters.
• We could add hide the <select> behind a custom label (name + arrow), where activating the label reveals the open select, and closing the select hides it, and reveals the updated label. The Duet datepicker does this.
• We could auto-resize the <select> based on the selected option. Example.

[airblade]

I went with the third option, in 3dbd1d4.

The down-arrows still aren't clickable.