Use theme page and header values if settings.js values are not present

[Steve-Mcl]

Types of changes

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

Proposed changes

• Permit a theme plugin to specify header and page values
• Only use theme page and header values if settings.js values are not present
• If neither are present, then NR defaults are used (as per v2.x)
• If settings.js values are present, use them (as per v2.x)

Tested...

settings.js	theme	Results in...
page.favicon = null	page.favicon = null	uses default NR favicon
page.favicon = null	page.favicon = 'common/favicon.ico'	uses theme favicon
page.favicon = 'custom-icon.ico'	page.favicon = 'common/favicon.ico'	uses settings.js custom favicon
.	.	.
page.title = null	page.title = null	uses default NR title
page.title = null	page.title = 'theme title'	uses theme title
page.title = 'settings.js title'	page.title = 'theme title'	uses settings.js title
.	.	.
header.image = null	header.image = null	uses default NR image
header.image = null	header.image = 'common/image.ico'	uses theme image
header.image = 'custom-icon.ico'	header.image = 'common/image.ico'	uses settings.js custom image
.	.	.
header.title = null	header.title = null	uses default NR title
header.title = null	header.title = 'theme title'	uses theme title
header.title = 'settings.js title'	header.title = 'theme title'	uses settings.js title
.	.	.
header.url = null	header.url = null	uses default NR url
header.url = null	header.url = 'theme url'	uses theme url
header.url = 'settings.js url'	header.url = 'theme url'	uses settings.js url

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 68.186% when pulling 8bd02d0 on Steve-Mcl:use-theme-page-and-header-values into b28595c on node-red:master.