Link various sign up buttons to /signup

app/soapbox/features/ui/components/navbar.tsx

@@ -85,7 +85,7 @@ const Navbar = () => {
                 </Button>
 
                 {!singleUserMode && (
-                  <Button theme='primary' to='/' size='sm'>
+                  <Button theme='primary' to='/signup' size='sm'>
                     <FormattedMessage id='account.register' defaultMessage='Sign up' />
                   </Button>
                 )}

app/soapbox/features/ui/components/panels/sign-up-panel.tsx

@@ -23,7 +23,7 @@ const SignUpPanel = () => {
         </Text>
       </Stack>
 
-      <Button theme='primary' block to='/'>
+      <Button theme='primary' block to='/signup'>
         <FormattedMessage id='account.register' defaultMessage='Sign up' />
       </Button>
     </Stack>

app/soapbox/features/ui/components/unauthorized_modal.js

@@ -102,7 +102,7 @@ class UnauthorizedModal extends ImmutablePureComponent {
   onRegister = (e) => {
     e.preventDefault();
 
-    this.props.history.push('/');
+    this.props.history.push('/signup');
     this.onClickClose();
   }