Added new environment variables for custom css, custom title, and custom description

[HrBingR]

Fixes #89

Fully tested using the Dockerfile to build an image and deployed via docker using the following environment variables (sensitive data redacted):

      BASE_URL: 'https://URL'
      NODE_VERSION: '16.13.1'
      YARN_VERSION: '1.22.15'
      PORT: '1443'
      MAX_FILE_SIZE: '1073741824'
      MAX_FILES_PER_ARCHIVE: '20'
      MAX_EXPIRE_SECONDS: '86400'
      MAX_DOWNLOADS: '5'
      UI_COLOR_PRIMARY: '#CC0000'
      UI_COLOR_ACCENT: '#990000'
      UI_CUSTOM_ASSETS_ICON: 'custom_assets/logo-black.png'
      UI_CUSTOM_ASSETS_WORDMARK: 'custom_assets/logo-na.png'
      CUSTOM_TITLE: 'FileShare'
      CUSTOM_DESCRIPTION: 'Send encrypted files securely over the internet'
      UI_CUSTOM_CSS: 'custom_assets/custom_css.css'

Added the ability for a user to define and use a custom CSS file.

For myself the only thing I changed in my custom CSS file is the background-image, but any CSS in there should overwrite the default CSS as it's been placed lower in the layout.js file.

I've also added the ability for the user to change the website title and description using environment variables to afford more customizability.

A production example can be found on https://fileshare.thenextweb.co.za with all three of these changes made.

Happy to post the full docker-file if needed.

[timvisee]

Thanks for these changes. These are very useful.

Would you mind to resolve the comment I just posted. After that I'll happily merge.

An if-statement should be used here, to only include this if a custom stylesheet is set (not null or empty string).

[timvisee]

LGTM. Thanks!