Add cookies to the web app

Extensively implemented cookies into the web app.
- Every time `setValue` or `doSpecialButtonAction` is called, the `gridState` cookie is updated with the current grid values, classes, direction, cell coordinates and crossword UUID (this is done with the new `saveGridState` method).
- A similar method, `saveToggleState` has also been implemented.

Removed some hard-coded strings:
- Grid operation related strings (moved into the `gridOp` object)
- Operation magnitude related strings (moved into the `opMagnitude` object)

Added a minor ergonomic feature - pressing [Shift+Backspace] with `auto-word` on when a word is already cleared will shift your word selection backwards.

README.md

@@ -78,6 +78,7 @@ Check out the wiki [here](https://github.com/tomasvana10/crossword_puzzle/wiki)
 - [NYTimes Mini Crossword](https://www.nytimes.com/crosswords/game/mini) - Heavily inspired the design of the web application
 - [CSS Pattern](https://css-pattern.com) - Background CSS patterns
 - [Pure CSS Toggle Switch](https://codepen.io/morgoe/pen/VvzWQg) - Toggle switch CSS patterns
+- [W3Schools](https://www.w3schools.com/js/js_cookies.asp) - Cookie getting/setting code
 - Crossword completion sound effect (CC attribution):
   - Jazzy Chords by NenadSimic -- https://freesound.org/s/150879/ -- License: Creative Commons 0