URL cleanup in admin center

Registered by Sander

Currently, pluck uses a lot $_GET variables (specified in data/inc/post_get.php). Pluck uses for every feature an own variable, for example:
 - admin.php?editmeta=something
 - admin.php?editpage=something
This results in a lot of variables. We should clean it up a little, so that the above URLs become:
 - admin.php?action=editmeta&var1=something
 - admin.php?action=editpage&var1=something

Blueprint information

Status:
Complete
Approver:
Sander
Priority:
Medium
Drafter:
Sander
Direction:
Approved
Assignee:
Sander
Definition:
Approved
Series goal:
Accepted for 4.7
Implementation:
Implemented
Milestone target:
milestone icon 4.7-beta
Started by
Sander
Completed by
Anders G. Jørgensen

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