Message Search Handler Concept should support search in *all* public channels

[tkurz]

Currently the search handler results are filtered via room subscriptions. That means: I cannot find things that I could actually see. The extension should allow a boolean configuration for search: include all open channels. This should be disabled by default.

The implementation must be placed in the rocketchat/search module. It has to be checked if such configurations can be done globale (not for every search provider).