Improve member adding and excluding flow

Allow to exclude a member

Send emails to the member when it's excluded

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/templates/email/group_invite.text.eex

@@ -1,5 +1,6 @@
 <%= gettext "Come along!" %>
 ==
 <%= gettext "%{inviter} just invited you to join their group %{group}", group: @group.name, inviter: @inviter.name %>
+<%= @group.url %>
 <%= gettext "To accept this invitation, head over to your groups." %>
 <%= page_url(Mobilizon.Web.Endpoint, :my_groups) %>