A bunch of tiny little fixes for issues I found while setting up docusaurus (#312)

* Do not show users related UI if no users have been specified

* Fixed typo in Docusaurus homepage link

* Allow projects without icons

Not every project has an icon, so avoid showing a broken image in that case.

* Do not show project icon in the footer if there's no footer icon

lib/core/nav/HeaderNav.js

@@ -188,10 +188,12 @@ class HeaderNav extends React.Component {
         <div className="headerWrapper wrapper">
           <header>
             <a href={this.props.baseUrl}>
-              <img
-                className="logo"
-                src={this.props.baseUrl + siteConfig.headerIcon}
-              />
+              {siteConfig.headerIcon && (
+                <img
+                  className="logo"
+                  src={this.props.baseUrl + siteConfig.headerIcon}
+                />
+              )}
               {!this.props.config.disableHeaderTitle && (
                 <h2 className="headerTitle">{this.props.title}</h2>
               )}