Update labels and functionality on configuration page

[andrewholgate]

The following default should be used on the form:

Path to XML sitemap

• [x] Change field label: "XML Sitemap file path or URL"
• [x] Add field description: "Enter a file path such as /sitemap.xml or a URL such as http://example.com/sitemap.xml"
• [x] Value should be prepopulated depending on the default directory of the XML Sitemap module installed on the site:
  • If XML Sitemap or Simple XML Sitemap is installed, use their default URI for the sitemap.xml files. For example, Simple XML Sitemap uses the path /sitemap.xml
• [x] The field should allow for non-absolute URI's, eg. /sitemap.xml & sitemap.xml should be acceptable, as well as http://example.com/sitemap.xml

  Downloaded HTML sitemaps directory name or path

• [x] Change field label: "HTML download directory"
• [x] Add field description: "Sub-directory that HTML pages are download into in the files/ directory"

  Reported HTML sitemaps directory name or path

• [x] Change field label: "Report download directory"
• [x] Add field description: "Sub-directory where reports are generated in the files/ directory"

  Audit pages modified since

• [x] Append the text "hours" to the end of the field. (as mentioned in #20)
  • [field] hours
• [x] Add field description: "Only audit pages which have been modified within this many hours."

  Accessibility standard

• [x] Include missing Section508 (see allowedStandards for the complete list.
• [x] Default standard should be: WCAG2AA
• [x] Add field description: "The accessibility standard to use when testing pages."

  Ignore accessibility levels

• [x] Change field label: "Accessibility reporting level" We need to flip the logic of this so that it reports on the level which are selected and does not report on the levels which are not selected.
• [x] Add field description: "The level of message to fail on"
• [x] The default selected values should be: warning and error

  HTML5 audit errors only

• [x] Add a title above it with the text "HTML5 audit"

  Link audit report verbose

• [x] Add a title above it with the text "Link audit"
• [x] Change checkbox label to: "Verbose Link audit report"