Add plugin install scripts to network dashboard, improve iframe position

[ocean90]

The patch

• takes the admin bar height into account, like r16681
• adds plugin-install style and script to network admin dashboard
• uses tb_position as the function name to override the original tb_position function from thickbox, it prevents layout issues.

[dd32]

Quick note, I've dealt with the JS errors over in #15781 & [16881]

[nacin]

(In [16963]) Plugin install and thickbox for the network admin. props ocean90, fixes #15510, fixes #15750.