Add initial support for custom stylesheets (#7164)

* Add initial support for custom stylesheets

Allow an override file `custom.css` that can partially override the
stylesheet used for the web interface.

# Changes
1. Update default views to use custom stylesheet
2. Update documentation

Related to: https://github.com/Ylianst/MeshCentral/issues/7144

* Add custom style support to more views

Load `custom.css` in the following views:
- agentinvite.handlebars
- invite.handlebars
- login2.handlebars
- message2.handlebars
- messenger.handlebars
- player.handlebars
- sharing.handlebars

views/messenger.handlebars

@@ -9,6 +9,7 @@
     <meta name="robots" content="noindex,nofollow">
     <link type="text/css" href="styles/style.css" media="screen" rel="stylesheet" title="CSS" />
     <link type="text/css" href="styles/messenger.css" media="screen" rel="stylesheet" title="CSS" />
+    <link type="text/css" href="styles/custom.css" media="screen" rel="stylesheet" title="CSS" />
     <link rel="apple-touch-icon" href="/favicon-303x303.png" />
     <script type="text/javascript" src="scripts/common-0.0.1{{min}}.js"></script>
     <script type="text/javascript" src="scripts/filesaver.min.js"></script>