Add styling for contact cards

[charludo]

Is your feature request related to a problem? Please describe.
Once digitalfabrik/integreat-cms#3169 is merged, regions will be able to insert contact cards into pages. While the CMS/api will provide the HTML markup for the card ready-to-use, due to the reasons discussed here and here, styling should be handled by the app.

Feel free to either copy the styling from the CMS (kudos @PeterNerlich):

.contact-card {
    display: inline-block;
    box-sizing: border-box;
    min-width: 50%;
    padding: 0.1em 1em;
    border-radius: 0.3em;
    background: rgba(127, 127, 127, 0.15);
    box-shadow: 0 .1em .1em rgba(0,0,0,0.4);
    cursor: default !important;
    color: initial;
    text-decoration: initial;
    background-image: linear-gradient(to right, rgba(255,255,255,0.9) 0 100%), url({% get_static_prefix %}/svg/contact.svg) !important;
    background-blend-mode: difference;
    background-position: calc(100% + 2em) calc(100% + 1em);
    background-size: 7em;
    background-repeat: no-repeat;
}

or to come up with your own web/app specific styling 😄

Describe the solution you'd like
Add styling for the contact-card class on web and mobile.

Describe alternatives you've considered
None. Styling in the CMS comes with some hurdles we'd rather avoid.

Additional context
CMS issue: digitalfabrik/integreat-cms#2959
Each .contact-card will contain a .marker-link which should receive display: none as its styling.