Memberships: Adds Breadcrumb from Membership: Show page

I noticed when working on the
[Journal](https://github.com/zinc-collective/convene/issues/898) that
when you click on the `Account` link, it kind of drops you into the
middle of no where.

This adds a breadcrumb, so we can find our way back! Yay!

app/models/membership.rb

@@ -15,6 +15,10 @@ class Membership < ApplicationRecord
 
   validates :member, uniqueness: { scope: :space_id }
 
+  def member_name
+    member.display_name
+  end
+
   enum status: {
     active: 'active',
     revoked: 'revoked'

app/views/memberships/show.html.erb

@@ -1,3 +1,4 @@
+<%- breadcrumb :show_membership, membership %>
 <h1><%= membership.member.name %></h1>
 <p><%= membership.member.email %></p>
 
@@ -6,4 +7,4 @@
   <ul>
     <%= render membership.sent_invitations %>
   </ul>
-</section>
+</section>

config/breadcrumbs.rb

@@ -15,7 +15,16 @@
 
 crumb :memberships do |space|
   link 'Members', [space, :memberships]
-  parent :edit_space, space
+  if policy(space).edit?
+    parent :edit_space, Space
+  else
+    parent :root
+  end
+end
+
+crumb :show_membership do |membership|
+  link membership.member_name, [membership.space, membership]
+  parent :memberships, membership.space
 end
 
 crumb :invitations do |space|
@@ -59,4 +68,4 @@
 crumb :rsvp do |rsvp|
   link 'Respond to your Invitation'
   parent :root, rsvp.space
-end
+end