Reorganize imports and add eslint rules

[saturninoabril]

Summary

Sort/order imports by the following groupings separated by empty line:

• jquery (retain on top to remind that this should remove whenever possible)
• built-in modules
• react/redux and related modules
• other dependency modules
• dev dependency modules
• mattermost modules (marked/mattermost-redux)
• non-npm dependency modules
• local imports ["actions", "dispatcher", "reducers", "selectors", "store", "stores"]
• local imports ["client", "config", "plugins", "routes"]
• local imports ["utils", "tests"]
• local imports ["fonts", "i18n", "images", "sass"]
• local imports ["components"]
• parentImports, starts with "../"
• siblingImports, starts with "./"

Sorting is done automatically via script: https://github.com/saturninoabril/mm-sort-imports

Add ESLint rules:

"import/no-unresolved": 2,
"import/order": ["error", {"newlines-between": "always-and-inside-groups", "groups": ["builtin", "external", ["internal", "parent"], "sibling", "index"]}],

Other minor changes:

• Add Mattermost copyright when not provided
• Remove/fix unresolved/unused modules
• move const declaration after all import statements

Note:

• Imports sort/order is not implemented to ./root.jsx

Ticket Link

none

Checklist

• Touches critical sections of the codebase (imports)

[jwilander]

Let's wait until after 4.3 code complete to merge this

[saturninoabril]

done rebase and fixed conflicts