Ultimate Fix Proposal: Vendor CSS issue during build & compile

[TheReluctantHeroes]

I noticed that there are a bunch discussions about issues relevant to organizing vendor CSS via grunt build & compile tasks but all PRs as far as I can see did not get the point and makes the situation worse.

Thus I raised this PR. Should be able to solve issues mentioned in
#340
#305
67739b8

My PR is implemented based on discussion in #185

As far as I know the correct way of loading external Vendor CSS is that
we define it in build.config.js->vender_files->css (other than
explicitly import it in main.less, etc); however there is something
wrong with loading vendor css via grunt tasks in commit
268901b.

Weird Behavior before this fix:

• vendor CSS will be concatenated(merged) instead of copied in build
• index.html still has reference to vendor CSS as standalone files in
  build
• vendor CSS will disappear if compile without build first because
  'copy:compile_assets' task simply copies concatenated all-in-one css
  file from build folder

Current Behavior after this fix:

• build no longer concatenates the vendor CSS while it now has correct,
  standalone reference in index.html to vendor CSS copied to its vendor
  folder
• compile has always vendor CSS concatenated (merged) and integrated
  regardless of whether build task has been performed

The modification has been tested under following cases:

• watch (delta)
• default (build & compile)
• compile after build
• compare w/o build

Still, please kindly notice:

• watch on LESS change will not notify to concatenate vendor CSS because
  they are not relevant in build
  From personal point of view I would not support following PRs, which did not suffice to be merged in the main branch:
  re-concat CSS files when LESS files change #369
  add concatenating css on less change #390
• Regardless of what's mentioned in the documentation, compile task
  still partially depends on build task for javascript files in src/ while
  all vendor files (incl. CSS, JS) can be compiled successfully all the
  time. This is due to historical reasons that 'ngAnnotate' and
  'concat:compile_js' loads file from build folder. As I'm quite unsure if
  there exists modification during the process of copying those relevant
  app js from original src to build, I just leave it remained as for now.