feat: add linked accounts page (#883)

* feat(linked-accounts): create page and display linked media server accounts

* feat(dropdown): add new shared Dropdown component

Adds a shared component for plain dropdown menus, based on the headlessui Menu component. Updates
the `ButtonWithDropdown` component to use the same inner components, ensuring that the only
difference between the two components is the trigger button, and both use the same components for
the actual dropdown menu.

* refactor(modal): add support for configuring button props

* feat(linked-accounts): add support for linking/unlinking jellyfin accounts

* feat(linked-accounts): support linking/unlinking plex accounts

* fix(linked-accounts): probibit unlinking accounts in certain cases

Prevents the primary administrator from unlinking their media server account (which would break
sync). Additionally, prevents users without a configured local email and password from unlinking
their accounts, which would render them unable to log in.

* feat(linked-accounts): support linking/unlinking emby accounts

* style(dropdown): improve style class application

* fix(server): improve error handling and API spec

* style(usersettings): improve syntax & performance of user password checks

* style(linkedaccounts): use applicationName in page description

* fix(linkedaccounts): resolve typo

* refactor(app): remove RequestError class

src/assets/services/jellyfin-icon.svg

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- ***** BEGIN LICENSE BLOCK *****
+  - Part of the Jellyfin project (https://jellyfin.media)
+  - 
+  - All copyright belongs to the Jellyfin contributors; a full list can
+  - be found in the file CONTRIBUTORS.md
+  - 
+  - This work is licensed under the Creative Commons Attribution-ShareAlike 4.0 International License.
+  - To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/4.0/.
+- ***** END LICENSE BLOCK ***** -->
+<svg version="1.1" id="icon-transparent" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 512 512">
+	<defs>
+		<linearGradient id="linear-gradient" gradientUnits="userSpaceOnUse" x1="110.25" y1="213.3" x2="496.14" y2="436.09">
+			<stop offset="0" style="stop-color:#AA5CC3"/>
+			<stop offset="1" style="stop-color:#00A4DC"/>
+		</linearGradient>
+	</defs>
+	<title>icon-transparent</title>
+	<g id="icon-transparent">
+		<path id="inner-shape" d="M256,201.6c-20.4,0-86.2,119.3-76.2,139.4s142.5,19.9,152.4,0S276.5,201.6,256,201.6z" fill="url(#linear-gradient)"/>
+		<path id="outer-shape" d="M256,23.3c-61.6,0-259.8,359.4-229.6,420.1s429.3,60,459.2,0S317.6,23.3,256,23.3z
+		M406.5,390.8c-19.6,39.3-281.1,39.8-300.9,0s110.1-275.3,150.4-275.3S426.1,351.4,406.5,390.8z" fill="url(#linear-gradient)"/>
+	</g>
+</svg>