Fix embedded SVG fill attribute (#4086)

* Fix embedded SVG fill attribute

SCSS darken/lighten functions may not return a color value, but a color
name like "white".  See following example:

https://www.sassmeister.com/gist/c41da93b87d536890ddf30a1f42e7816

This patch will normalize $color argument to FFFFFF style.

I also changed the function name from "url-friendly-colour" to
"hex-color", Because...

1. The name "url-friendly" is not meaningful enough to describe what it
   does.

2. It is familier to me using "color" rather than "colour"

    kojima:kojiMac mastodon[master]$ git grep -l colour
    app/javascript/styles/boost.scss
    spec/fixtures/files/attachment.jpg

    kojima:kojiMac mastodon[master]$ git grep -l color
    .rspec
    .scss-lint.yml
    Gemfile.lock
    app/javascript/mastodon/features/status/components/action_bar.js
    app/javascript/styles/about.scss
    app/javascript/styles/accounts.scss
    app/javascript/styles/admin.scss
    app/javascript/styles/basics.scss
    app/javascript/styles/boost.scss
    app/javascript/styles/compact_header.scss
    app/javascript/styles/components.scss
    app/javascript/styles/containers.scss
    app/javascript/styles/footer.scss
    app/javascript/styles/forms.scss
    app/javascript/styles/landing_strip.scss
    app/javascript/styles/reset.scss
    app/javascript/styles/stream_entries.scss
    app/javascript/styles/tables.scss
    app/javascript/styles/variables.scss
    app/views/admin/subscriptions/_subscription.html.haml
    app/views/layouts/application.html.haml
    app/views/layouts/error.html.haml
    app/views/manifests/show.json.rabl
    bin/webpack-dev-server
    config/initializers/httplog.rb
    public/500.html
    public/emoji/1f1e6-1f1e8.svg
    public/emoji/1f1ec-1f1f8.svg
    public/emoji/1f1f3-1f1ee.svg
    public/emoji/1f1fb-1f1ec.svg
    spec/fixtures/requests/idn.txt
    yarn.lock

* Add semicolon