Sip protocol not supported in Web Chat v4

[tdurnford]

Description

I noticed Web Chat v3 supports the sip protocol while investigating IcM #124908718; however, it is not supported in Web Chat v4.

Steps to Reproduce

Send an activity with a sip link to Web Chat.

await context.sendActivity('[Sip Link](sip:[email protected])');

Expected Result / v3 Result

<p><a href="sip:[email protected]" target="_blank">Sip Link</a></p>

Actual Result / v4 Result

<p><a target="_blank">Sip Link</a></p>

Fix

We can add the sip protocol to either the allowedSchemes or allowedSchemesByTag attributes in Web Chat's sanitize html options to resolve the issue (sanitize html default options).

const SANITIZE_HTML_OPTIONS = {
  ...
  allowedSchemes : ['http', 'https', 'ftp', 'mailto', 'sip'],
  ... and/or
  allowedSchemesByTag: { a: ['http', 'https', 'ftp', 'mailto', 'sip'] }
};

Additional context

Directly related to issue #199

[Bug]