Fix long menu when hidden by footer bar

[yeremi]

Fixes #18

When the hamburger menu has numerous items it's hidden by the footer bar.

As described at https://www.w3schools.com/cssref/pr_pos_overflow.asp, the overflow property only works for block elements with a specified height.

Then, we set the SHM height considering the viewport height less footer bar height, avoiding hide footer bar and allowing scrolling on SHM.

Screenshots

How to test the changes in this Pull Request:

1. Install and activate Woocommerce plugin, Storefront Hamburger Menu plugin, and Storefront theme;
2. Add many options to the menu;
3. Open a browser window on a mobile device;
4. Open the hamburger menu and scroll by the menu.

Changelog

Fix long menu when hidden by footer bar

Tests

• I've tested browser support to ensure compatibility with >= IE11
  Chrome on Samsung Galaxy S6 v5.0
  Chrome on Samsung Galaxy S7 v6.0
  Chrome on Samsung Galaxy S9 v8.0
  Safari on iPhone 7 v10.3
  Safari on iPhone 8 v12.1
  Safari on iPhone 8 v13.5
  Chrome on Xiaomi Redmi Note 7 v9.0
  Chrome on Xiaomi Redmi Note 8 v9.0

• I've tested using only a keyboard (no mouse)

• I've tested using a screen reader

• All animations respect prefers-reduced-motion

• All text has at least a 4.5 color contrast with its background