Allowing to use alternative search functions (nodebb-elasticsearch-plugin)

An admin setting for posts to disable DB search. If disableDbSearch is set,  the route /search/:term (only this one) is not added in api.js.
The plugin can now register /api/search/:term  to perform the search. 
It would be nice if the plugin could set disableDbSearch when it gets installed/activated (e.g. before.app.loaded)