Added Guarda wallet to Downloads page (with closed source warning)

Guarda Wallet has been asking me to add them to the Monero site since their wallet supports Monero.

Only parts of their wallet are open sourced, which is basically the same as the wallet being closed-source, so I added it with a warning. Does anyone disagree with merging this? Should we only point people to FOSS solutions?

css/custom.css

@@ -3020,6 +3020,22 @@ footer {
     padding: 0.5rem;
 }
 
+.downloads .mob-wallets a.closed-src::after {
+    content: 'Closed Source. Use with Caution.';
+    display: block;
+    background-color: #e5ba38;
+    border-radius: 2px;
+    width: 8rem;
+    margin-left: auto;
+    margin-right: auto;
+    font-size: 0.6rem;
+    line-height: 1.2;
+    text-align: center;
+    overflow-wrap: normal;
+    padding: 0.2rem 0;
+    margin-top: -0.4rem;
+}
+
 .downloads .full>.info-block {
     padding: 4rem 5rem;
 }