[Auth] Add LaunchDarkly to custom icons (#6399)

## Description

Adds a custom icon for the LaunchDarkly feature flag/automation service.
Icon was taken from their [press
kit](https://launchdarkly.com/press-and-analysts/) and given a hex code
to work on both dark & light modes.

## Tests

n/a

auth/assets/custom-icons/_data/custom-icons.json

@@ -574,7 +574,7 @@
       "title": "IBKR",
       "slug": "ibkr",
       "altNames": [
-        "Interactive Brokers", 
+        "Interactive Brokers",
         "IB"
       ]
     },
@@ -699,6 +699,10 @@
         "飞书"
       ]
     },
+    {
+      "title": "LaunchDarkly",
+      "hex": "858585"
+    },
     {
       "title": "Letterboxd"
     },

auth/assets/custom-icons/icons/launchdarkly.svg

@@ -0,0 +1 @@
+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 216 214.94"><path d="M109.8,214.94a4.87,4.87,0,0,1-4.26-2.66,4.5,4.5,0,0,1,.44-4.82l50.49-69.53L68,174.11a4.61,4.61,0,0,1-1.9.41,4.77,4.77,0,0,1-4.52-3.4,4.57,4.57,0,0,1,2-5.21L141.33,120,4.41,112.13a4.69,4.69,0,0,1,0-9.36l137-7.87L63.61,49a4.56,4.56,0,0,1-1.94-5.2,4.74,4.74,0,0,1,4.51-3.4,4.6,4.6,0,0,1,1.9.4L156.5,77,106,7.48a4.56,4.56,0,0,1-.44-4.83A4.84,4.84,0,0,1,109.84,0a4.59,4.59,0,0,1,3.28,1.41L213.77,102.05a7.65,7.65,0,0,1,0,10.8L113.08,213.53A4.59,4.59,0,0,1,109.8,214.94Z"/></svg>