Reverting any single of the scripts from standard recommendation pool
shows "Standard" selection as selected which is wrong. This commit fixes
it, refactors selection handling in a separate class and it also adds
missing tests. It removes UserSelection.totalSelected propertty in favor of using
UserSelection.selectedScripts.length to provide unified way of accessing
the information.
Alias would remove unnecessary repetitions and less relative paths make changes easier when moving around files. This commit cleans also up some relative paths ('../../../') by using the alias and orders imports. It updates both path alias in tsconfig and module alias in Vue CLI's bundler (vuejs/vue-cli#2398).
