Footer share link

Added mastodon and mattermost icon, and edited them to the color that the icons had. Added the link to the Public:Town Square. Edited the app.css but the changes are not reflected in this merge requester and I don't know why. If the icon doesn't look like this,

then change at app.css #footer .footer-social-link li a { height: 35px; width: 35px; line-height: 35px;

because the parameters already are 45 px Issue #330