use cp theme for welcome message overlay, use build array #2675

#### Changes
* defined variable for welcome message menu path
* changed path to use "cp/" prefix (in order to use cp theme)
* use build array to generate message
cleanup: comments, refactor, rename #2675

#### Changes
* rename path to 'cp/welcome'
* comment everywhere
* rename variables
* format code
* separate logic from assignment
Merge pull request #2940 from openscholar/2675-ctools-modal

standardize overlay style using CP theme for site welcome message #2675