Warn on empty `ADMIN_TOKEN` instead of bailing out

The admin page will still be disabled.

Fixes #849.

src/api/admin.rs

@@ -17,7 +17,7 @@ use crate::mail;
 use crate::CONFIG;
 
 pub fn routes() -> Vec<Route> {
-    if CONFIG.admin_token().is_none() && !CONFIG.disable_admin_token() {
+    if !CONFIG.disable_admin_token() && !CONFIG.is_admin_token_set() {
         return routes![admin_disabled];
     }
 

src/config.rs

@@ -430,7 +430,8 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
 
     if let Some(ref token) = cfg.admin_token {
         if token.trim().is_empty() && !cfg.disable_admin_token {
-            err!("`ADMIN_TOKEN` is enabled but has an empty value. To enable the admin page without token, use `DISABLE_ADMIN_TOKEN`")
+            println!("[WARNING] `ADMIN_TOKEN` is enabled but has an empty value, so the admin page will be disabled.");
+            println!("[WARNING] To enable the admin page without a token, use `DISABLE_ADMIN_TOKEN`.");
         }
     }
 
@@ -617,6 +618,13 @@ impl Config {
         }
     }
 
+    /// Tests whether the admin token is set to a non-empty value.
+    pub fn is_admin_token_set(&self) -> bool {
+        let token = self.admin_token();
+
+        !token.is_none() && !token.unwrap().trim().is_empty()
+    }
+
     pub fn render_template<T: serde::ser::Serialize>(
         &self,
         name: &str,