Feature: allow defining of settings capability for more granular access control

[spenserhale]

What

Introduces optional constant WP_STREAM_SETTINGS_CAPABILITY to allow site owners to set more granular control over who can manage Stream's settings. It defaults to the existing manage_options for backward compatibility.

Why

We run a large multisite and request the ability to have a custom capability name so we can filter access to Stream settings for our network and site admins based on user settings.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).

[marcinkrzeminski]

@spenserhale thank you for contributing.

Here's a link to the related enhancement proposed https://github.com/hokoo by #1553