Folder picker modal for file imports

js/api.js

js/store/folderpicker.js

js/store/index.js

scss/main.scss

scss/tree.scss

test/store/actions.js

test/store/folderpicker.js

test/store/index.js

test/testhelpers.js

vue/Imports/Files/ImportFromFiles.vue

vue/Imports/Files/Uploader.vue

vue/Navigation/ChildrenTree/ChildrenTree.vue

vue/Navigation/ChildrenTree/TreeItem.vue

vue/Navigation/FolderPicker/Folder.vue

vue/Navigation/FolderPicker/FolderList.vue

vue/Navigation/FolderPicker/FolderPicker.vue

vue/Navigation/FolderPicker/index.js