Do not use Watchlist store to get watchers of a page

Using the store is way too much overhead for what we need here, leading to
memory issues on big wikis
Testing users after retrieving it is still quite expensive, but should be ok

[4.3.x]

ERM32862

Change-Id: I62ca428f629d5e05e9eacdd20f3f04bbb60aae6f