Clean up gulpfile.js, refs T1067

Summary:
- style changes (trailing commas, newlines)
- clarify createDefaultTaskDependencies
- remove watchify, replace with `gulp.watch` and browserify
- optimize dependencies

Test Plan: `gulp`

Reviewers: spaudanjo, rjmackay

Subscribers: seth

Differential Revision: https://phabricator.ushahidi.com/D575
use ng-href instead of href
delete unused user-entity.js
fix type in post-endpoint
extract user-endpoint to own common module (because it's a ressource which will be used by several other parts/modules of the app)