UPDATE: Adds web app (#79)

Things missing:

* Browse mode
* Front end tests
* Storybook

base/src/test/user_model/mod.rs

@@ -1,14 +1,20 @@
 mod test_add_delete_sheets;
 mod test_autofill_columns;
 mod test_autofill_rows;
+mod test_border;
 mod test_clear_cells;
 mod test_diff_queue;
 mod test_evaluation;
 mod test_general;
 mod test_grid_lines;
+mod test_keyboard_navigation;
+mod test_on_area_selection;
+mod test_on_expand_selected_range;
+mod test_on_paste_styles;
 mod test_rename_sheet;
 mod test_row_column;
 mod test_styles;
 mod test_to_from_bytes;
 mod test_undo_redo;
 mod test_view;
+mod test_window_size;