let a link be used by several people instead of dying on first open

The create popup now has a "Let several people use this link" box next to the
expiry controls. Tick it and the link stays redeemable by everyone you send it to
until it expires; leave it and you get the old behaviour, where the first person
to open it is the only one who gets in. The plugin setting that used to be the
only control is now just what the box starts out as, and its label on the config
page says so, because "Default one-use links" explained nothing.

Multi-use did not actually work before this. Two things in Jellyfin stopped it,
and both had to change:

Guests were given MaxActiveSessions = 1, and AuthenticateNewSessionInternal throws
SecurityException once a user is at that limit. The second viewer's redemption
would fail, the record would go to Failed, and cleanup would then delete the guest
account, kicking the first viewer out too. Multi-use links now get 0, which is how
Jellyfin spells "no limit" in that check.

The device id was generated once and reused for every redemption, and
GetAuthorizationToken logs out every existing session for the same user and device
before issuing a token. So even under a raised session cap, each new viewer would
have revoked the previous one's token. Multi-use links now mint a device id per
redemption.

Both viewers of a multi-use link share one temporary account, so they also share
playback position and watched state on the shared title.

Jellyfin.Plugin.ShareLinks/Web/configPage.html

@@ -101,8 +101,9 @@
                         <div class="checkboxContainer checkboxContainer-withDescription">
                             <label>
                                 <input is="emby-checkbox" type="checkbox" id="OneUseDefault" />
-                                <span>Default one-use links</span>
+                                <span>New links are single use by default</span>
                             </label>
+                            <div class="fieldDescription checkboxFieldDescription">This only decides how the "Let several people use this link" box starts out in the create popup; you can change it for every link you make. A single-use link stops working the moment the first person opens it, and only that person keeps access until it expires. A multi-use link can be opened by everyone you send it to, for as long as it is valid.</div>
                         </div>
 
                         <div class="sl-field inputContainer">