[meta] Clean up frontend import statements

We are currently using require.context() in place of all of the old require_directory and require_tree sprockets directives. These were necessary while migrating to webpack but now that it has been merged we can refactor these bad practice statements out.

Here's a checklist of the places where this is currently used:

• application.js
• boards/boards_bundle.js.es6
• cycle_analytics/cycle_analytics_bundle.js.es6
• diff_notes/diff_notes_bundle.js.es6
• filtered_search/filtered_search_bundle.js
• graphs/graphs_bundle.js
• network/network_bundle.js
• profile/profile_bundle.js
• protected_branches/protected_branches_bundle.js
• snippet/snippet_bundle.js
• users/users_bundle.js