Add support for CSP headers

Problem/Motivation

Piwik PRO has the possibility to support CSP headers. This functionality would be good to be implemented in the module also.

Steps to reproduce

TBD.

Proposed resolution

We should add a new configuration for the module to enable CSP and implement the nonce settings according to the Piwik PRO documentation.

Remaining tasks

1. Functionality
2. Documentation
3. Tests

User interface changes

A checkbox type option in the module page to enable CSP.

API changes

None.

Data model changes

A new boolean field to enable CSP.