[assets] Add rules to ESLint:

- Fix long line.
- Enforce a few stylistic habits:

    - Avoid some potential dangerous constructs.
    - `arrow-spacing`: Use at least one space around arrows.
    - `keyword-spacing`: Use at least one space around keywords (if, else, for…).
    - `no-multiple-empty-lines`: Only use one empty line between code.
	- `no-var`: Use `let` or `const` instead of `var`:
    - `padded-blocks`: Do not pad blocks.
    - `padding-line-between-statements`: Use empty lines between some statements.
    - `space-before-blocks`: Use at least one space before the opening brace of a block.

bookwyrm/static/js/localstorage.js

@@ -15,8 +15,9 @@ let LocalStorageTools = new class {
     // set javascript listeners
     updateDisplay(e) {
         // used in set reading goal
-        var key = e.target.getAttribute('data-id');
-        var value = e.target.getAttribute('data-value');
+        let key = e.target.getAttribute('data-id');
+        let value = e.target.getAttribute('data-value');
+
         window.localStorage.setItem(key, value);
 
         document.querySelectorAll('[data-hide="' + key + '"]')
@@ -25,8 +26,9 @@ let LocalStorageTools = new class {
 
     setDisplay(el) {
         // used in set reading goal
-        var key = el.getAttribute('data-hide');
-        var value = window.localStorage.getItem(key);
+        let key = el.getAttribute('data-hide');
+        let value = window.localStorage.getItem(key);
+
         BookWyrm.addRemoveClass(el, 'hidden', value);
     }
 }