Add support for multiple static paths with optional static root

[Steve-Mcl]

closes #3510

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Permits single or multiple static source paths and allows each of those to be served from a different root

By adding the httpStaticRoot we can avoid the need for changing httpAdminRoot when httpStatic is enabled.

Examples...

==========

Single path, default root

Settings File

    httpStatic: 'c:\\temp',
    //httpStaticRoot: '/',

Log Output

22 Apr 17:05:34 - [info] HTTP Static    : c:\temp > /

Multiple paths, alternative base root

Settings File

    httpStatic: [{path:'c:\\temp'}, {path:'c:\\tmp'}],
    httpStaticRoot: '/static_stuff/',

Log Output

22 Apr 17:05:34 - [info] HTTP Static    : c:\temp > /static_stuff/
22 Apr 17:05:34 - [info] HTTP Static    : c:\tmp > /static_stuff/

Multiple paths with default root and individual root

Settings File

    httpStatic: [{path:'c:\\temp', root: "/stuff/"}, {path:'c:\\tmp', root: "/other/"}],
    //httpStaticRoot: '/',

Log Output

22 Apr 17:05:34 - [info] HTTP Static    : c:\temp > /stuff/
22 Apr 17:05:34 - [info] HTTP Static    : c:\tmp > /other/

Multiple paths with base root and individual root

Settings File

    httpStatic: [{path:'c:\\temp', root: "/stuff/"}, {path:'c:\\tmp', root: "/other/"}],
    httpStaticRoot: '/static/',

Log Output

22 Apr 17:05:34 - [info] HTTP Static    : c:\temp > /static/stuff/
22 Apr 17:05:34 - [info] HTTP Static    : c:\tmp > /static/other/

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage decreased (-0.1%) to 68.655% when pulling 4715704 on Steve-Mcl:httpstatic-improvements into d802ce1 on node-red:dev.