Reduce replication of constants

[marvinchin]

Tell us about your environment

• MarkBind Version:
  MarkBind v1.16.1

What did you expect to happen?
Constants can be defined in a single file and reused where needed

What actually happened? Please include the actual, raw output.
Some constants are reused across Site.js, Page.js, and cliUtil.js (e.g. LAYOUT_FOLDER_PATH)

[nicholaschuayunzhi]

For different sub logic it has been suggested in #698 (comment) to have file naming structure of {{main module}}.{{sub module}}.js

Additionally it is suggested that constants should be placed in constants.js #698 (comment)