Fix no-useless-concat issues (#9200)

See [`no-useless-concat`](https://eslint.org/docs/rules/no-useless-concat) for more information.

This change enables `no-useless-concat` and fixes the issues raised by the rule.

.eslintrc.js

@@ -49,6 +49,7 @@ module.exports = {
     'guard-for-in': 'error',
     'no-case-declarations': 'error',
     'no-useless-catch': 'error',
+    'no-useless-concat': 'error',
     /* End v2 rules */
     'arrow-parens': 'error',
     'no-tabs': 'error',

ui/app/components/app/gas-customization/gas-price-chart/gas-price-chart.utils.js

@@ -263,10 +263,10 @@ export function generateChart (gasPrices, estimatedTimes, gasPricesMax, estimate
         let text
         d.forEach((el) => {
           if (el && (el.value || el.value === 0) && !text) {
-            text = "<table class='" + 'custom-tooltip' + "'>" + "<tr><th colspan='2'>" + titleFormat(el.x) + '</th></tr>'
+            text = `<table class='custom-tooltip'><tr><th colspan='2'>${titleFormat(el.x)}</th></tr>`
           }
         })
-        return text + '</table>' + "<div class='tooltip-arrow'></div>"
+        return `${text}</table><div class='tooltip-arrow'></div>`
       },
       position: function () {
         if (d3.select('#overlayed-circle').empty()) {

ui/app/components/app/modals/fade-modal.js

@@ -23,7 +23,7 @@ const insertRule = (css) => {
 const insertKeyframesRule = (keyframes) => {
   // random name
   const name = 'anim_' + (++index) + (+new Date())
-  let css = '@' + 'keyframes ' + name + ' {'
+  let css = `@keyframes ${name} {`
 
   Object.keys(keyframes).forEach((key) => {
     css += key + ' {'