invites: add "User Label" 1/2

Adds a "User Label" invite field, which is a label applied to users
created with it. This commit contains everything apart from the code to
apply it on account creation.

html/admin.html

@@ -589,6 +589,11 @@
                                 <label class="label supra" for="create-label"> {{ .strings.label }}</label>
                                 <input type="text" id="create-label" class="input ~neutral @low mb-2 mt-4">
                             </div>
+                            <div class="col">
+                                <label class="label supra" for="create-user-label"> {{ .strings.userLabel }}</label>
+                                <p class="support">{{ .strings.userLabelDescription }}</p>
+                                <input type="text" id="create-user-label" class="input ~neutral @low mb-2 mt-4">
+                            </div>
                         </div>
                         <div class="card ~neutral @low col">
                             <label class="label supra" for="create-uses">{{ .strings.inviteNumberOfUses }}</label>