referrals: add "use expiry" option

adds an option when enabling referrals to use the duration of the source
invited (i.e., months, days, hours) for the referral invite. If enabled,
the user won't be able to make a new referral link after it expires. For
referrals enabled for new users via a profile, the clock starts ticking
as soon as the account is created.

html/admin.html

@@ -130,6 +130,11 @@
                 <div class="select ~neutral @low mb-4 unfocused">
                     <select id="enable-referrals-user-invites"></select>
                 </div>
+                <label class="switch mb-4">
+                    <input type="checkbox" id="enable-referrals-user-expiry">
+                    <span>{{ .strings.useInviteExpiry }}</span>
+                    <span class="flex flex-row support mt-2">{{ .strings.useInviteExpiryNote }}</span>
+                </label>
                 <label>
                     <input type="submit" class="unfocused">
                     <span class="button ~urge @low full-width center supra submit">{{ .strings.apply }}</span>
@@ -144,6 +149,11 @@
                 <div class="select ~neutral @low mb-4 mt-2">
                     <select id="enable-referrals-profile-invites"></select>
                 </div>
+                <label class="switch mb-4">
+                    <input type="checkbox" id="enable-referrals-profile-expiry">
+                    <span>{{ .strings.useInviteExpiry }}</span>
+                    <span class="flex flex-row support mt-2">{{ .strings.useInviteExpiryNote }}</span>
+                </label>
                 <label>
                     <input type="submit" class="unfocused">
                     <span class="button ~urge @low full-width center supra submit">{{ .strings.apply }}</span>

html/user.html

@@ -155,7 +155,7 @@
                     <div>
                         <div class="card @low dark:~d_neutral unfocused" id="card-referrals">
                             <span class="heading mb-2">{{ .strings.referrals }}</span>
-                            <aside class="aside ~neutral my-4 col">{{ .strings.referralsDescription }}</aside>
+                            <aside class="aside ~neutral my-4 col user-referrals-description"></aside>
                             <div class="row flex-expand">
                                 <div class="user-referrals-info"></div>
                                 <div class="grid my-2">