Change query limit settings format to nested mappings

[AntonKhorev]

Instead of settings named like this Settings.max_changeset_query_limit
it will be easier to work with Settings.changesets.max_query_limit.

It matches the api capabilities response structure better.
It will be easier to create a generic result_limit method.

Also added settings validation in config/initializers/config.rb.

[tomhughes]

I'm not sure about this one I have to say - there are plenty of other settings that work as a group where we haven't done this.

[AntonKhorev]

I don't want to write something like Settings["max_#{type}_query_limit"] in a possible superclass method.

[AntonKhorev]

The idea is

• create a superclass for things searchable by the api; currently they are changesets and notes
• it's going to have result_limit method
• but result_limit reads type-specific settings
• then also add a method to read type-specific settings, rather than read them from the global Settings object; this is going to be either an abstract method to be defined in subclasses or derived from controller_name like in Support versions in elements multi fetch #3715