Implement GET /users/:uid/keys for admin users

Complements POST operation added in gitlabhq/gitlabhq#3146

Implement DELETE /users/:uid/keys/:id for admin users

Fix "Line is too long. [83/80]"

Use single quotes as advised

Use single quotes as advised

Use single quotes as advised

Fix missing space around { and }

Fix typo in documentation

Only catch  ActiveRecord::RecordNotFound, let other exceptions propagate

Raise a "404 Not found" if key to be deleted cannot be found
As requested by @jvanbaarsen in https://github.com/gitlabhq/gitlabhq/pull/6781#discussion_r11735114

Remove tab
Unconfigured vim on this box, grrrr./