Replace Brunch with webpack

Phoenix version 1.4 uses brunch in place of webpack. During the update , #124, of Phoenix 1.3 to 1.4 this step was skipped as it is optional.

Opening this issue as a place to discuss if we need to replace webpack.

Start by reviewing the Phoenix 1.3-to-1.4 update referenced as issue #124 and the repository’s current Brunch build setup. The issue names no files or tests and only proposes discussing a possible webpack migration, so first establish whether the replacement is needed and define the migration scope; it is done when that decision and scope are recorded.

Written by the indexing model from the issue text.