Admin tools development/Global CheckUser

It has been requested by numerous parties that the ability to run checkuser across multiple wiki simultaneously would enhance Stewards' ability to locate and block spam bots.

Requirements

 * Current local tool exists at m:Special:CheckUser; propose a parallel one at m:Special:GlobalCheckUser.
 * Accessed through new permissions ( and  ) - will be granted to the Stewards group, but should be a granular.
 * Will re-use local tool's code as much as possible to avoid code repetition/divergence.
 * Logged to a single global CU log, which will include all CUs, local and global.

Workflow

 * On m:Special:GlobalCheckUser, user uses tool as on m:Special:CheckUser.
 * Tool uses existing (where possible) functions to run CU on local wikis
 * Results are returned to a single UI on Meta

Questions

 * Where are log entries placed? Global CU log - Hard for local wikis to see the global action or Local CU log - Could potentially be a lot of log spam and cause many questions // could place in local log and identify them as global CU as an FYI
 * What does the global search results set look like? (Do we group by wiki, by action type, by IP, by day)?
 * Are smaller limits needed given the potential to slow all wikis down at once?