Shiny Updates: Installing Plugins

[jorbin]

Branched from #29820.

See ​https://make.wordpress.org/core/2015/03/25/dev-chat-agenda-march-25-2015/ and the associated dev chat

[jorbin]

In 31897:

Remove Shiny Plugin Installs

See #31773, #29820

[jorbin]

In 31900:

More clean up from removal of Shiny Plugin Installs

See #31773, #29820
Unprops Jorbin, Pento

[jorbin]

In 31901:

Remove Shiny Plugin Install strings

See #31773, #29820

[obenland]

In 37714:

Update/Install: Shiny Updates v2.

Gone are the days of isolation and feelings of "meh", brought on by The Bleak Screen of Sadness. For a shiny knight has arrived to usher our plugins and themes along their arduous journey of installation, updates, and the inevitable fate of ultimate deletion.

Props swissspidy, adamsilverstein, mapk, afragen, ocean90, ryelle, j-falk, michael-arestad, melchoyce, DrewAPicture, AdamSoucie, ethitter, pento, dd32, kraftbj, Ipstenu, jorbin, afercia, stephdau, paulwilde, jipmoors, khag7, svovaf, jipmoors, obenland.
Fixes #22029, #25828, #31002, #31529, #31530, #31773, #33637, #35032.