Frontend cleanups (#4449)

* Update some buttons

* Potential flaky test fix

* Dark theme: Fix primary accent color

* Pay Button: Remove unused clipboard dependency

The pay button uses the cope-to-clipboard.js

* Remove babel-polyfill

Browser-support should be good by now.

* Remove unused jquery-easing scripts

* Remove unused CSS

BTCPayServer.Tests/SeleniumTester.cs

@@ -192,7 +192,7 @@ namespace BTCPayServer.Tests
             Driver.SetCheckbox(By.Id("UseNewCheckout"), true);
             Driver.WaitForElement(By.Id("OnChainWithLnInvoiceFallback"));
             Driver.SetCheckbox(By.Id("OnChainWithLnInvoiceFallback"), bip21);
-            Driver.FindElement(By.Id("Save")).Click();
+            Driver.FindElement(By.Id("Save")).SendKeys(Keys.Enter);
             Assert.Contains("Store successfully updated", FindAlertMessage().Text);
             Assert.True(Driver.FindElement(By.Id("UseNewCheckout")).Selected);
         }