Improve UI to change Default Category

[kraftbj]

Spun off of #26268 per chat with @helen ​https://wordpress.slack.com/archives/core/p1424923581006944 #26268 will focus on UI indicating Default Category.

Currently, to change the Default Category, you must visit Settings->Writing. Since this only impacts one taxonomy, it may make more sense to remove the option from there and add it within the Category taxonomy.

Attaching modified patch of #26268 that contains only the UI to set the default.

[kraftbj]

Latest relevant patch from 26268. Props to kraftbj, paulwilde, seanchayes, wonderboymusic

[helen]

As a counterpoint, so that it's documented here - there are also options for default post format and default link category (if links are on). While those exist on that screen, does it make sense to split them?

[chriscct7]

Per https://core.trac.wordpress.org/ticket/32396#comment:5, the default post post format setting will probably start being hidden from new installs. Therefore, this should be reconsidered. Was talking with @kraftbj about this yesterday about doing this for 4.3