Submenus cannot exist without main menu relation

[xaralis]

When item is not in the main menu, corresponding pages cannot have submenu.

This can be easily fixed by refactoring submenu.html like following:

{% for menu in site.data.menu.submenus %}
  {% if page.url contains menu.match %}
    <nav class="c-top-sub-nav">
      <div class="row">
        <div class="c-top-sub-nav-inner columns">
          <div class="c-top-sub-nav__header">
            <h2 class="c-top-sub-nav__heading">{{ menu.name }}</h2>
          </div>
          <div class="c-top-sub-nav-divider"></div>
          <ul class="c-top-sub-nav-items">
            {% for submenu in menu.items %}
            <li class="c-top-sub-nav__item {% if page.url contains submenu.match %}c-top-sub-nav__item--active{% endif %}">
              <a href="{{ submenu.link }}">{{ submenu.name }} </a>
            </li>
            {% endfor %}
          </ul>
        </div>
      </div>
    </nav>
  {% endif %}
{% endfor %}

Then, in menu.yml, you can configure submenus separately like this:

submenus:
  - name: Volby 2018
    match: komunalni-volby-2018
    items:
      - link: /komunalni-volby-2018/
        name: Radnice
        match: komunalni-volby-2018
      - link: /komunalni-volby-2018/mo-i/
        name: MO I
        match: mo-i
      - link: /komunalni-volby-2018/mo-ii/
        name: MO II
        match: mo-ii
      - link: /komunalni-volby-2018/mo-iv/
        name: MO IV
        match: mo-iv
      - link: /komunalni-volby-2018/mo-v/
        name: MO V
        match: mo-v
      - link: /komunalni-volby-2018/mapa-piratskych-planu/
        name: Mapa pirátských plánů
        match: mapa-piratskych-planu
      - link: /komunalni-volby-2018/povolebni-strategie/
        name: Povolební strategie
        match: povolebni-strategie
      - link: /komunalni-volby-2018/kampan-dary-prispevky/
        name: Financování kampaně
        match: kampan-dary-prispevky

Unfortunately, this is a breaking change. Do we want it in the main repository, too?