Uses GitHub Avatar for GH 'noreply' email address

Partial fix for #281- covers the `*@users.noreply.github.com` case.

The `users.noreply.github.com` domain comes from users
making use GitHub's 'email privacy' feature.
Since the noreply address doesn't accept email, a user can't link
their preferred Gravatar to this type of email address.

The upside is that this address is easily parsable into:
* (optional) Github User ID
* Github User Name

We can use either to create the associated avatar link
without actually having to do an additonal request
against Github's user search API.

src/avatars.ts

@@ -15,6 +15,14 @@ export function clearGravatarCache() {
     gravatarCache.clear();
 }
 
+function getAvatarFromGithubNoreplyAddress(email: string | undefined, size: number = 16): Uri | undefined {
+    if (!email) return undefined;
+    const match = email.match(/^(?:(?<userId>\d+)\+)?(?<userName>[a-zA-Z\d-]{1,39})@users.noreply.github.com$/);
+    if (!match || !match.groups) return undefined;
+    const { userName, userId } = match.groups;
+    return Uri.parse(`https://avatars.githubusercontent.com/${userId ? `u/${userId}` : userName}?size=${size}`);
+}
+
 export function getGravatarUri(email: string | undefined, fallback: GravatarDefaultStyle, size: number = 16): Uri {
     const hash =
         email != null && email.length !== 0 ? Strings.md5(email.trim().toLowerCase(), 'hex') : missingGravatarHash;
@@ -23,7 +31,9 @@ export function getGravatarUri(email: string | undefined, fallback: GravatarDefa
     let gravatar = gravatarCache.get(key);
     if (gravatar !== undefined) return gravatar;
 
-    gravatar = Uri.parse(`https://www.gravatar.com/avatar/${hash}.jpg?s=${size}&d=${fallback}`);
+    gravatar =
+        getAvatarFromGithubNoreplyAddress(email, size) ||
+        Uri.parse(`https://www.gravatar.com/avatar/${hash}.jpg?s=${size}&d=${fallback}`);
     gravatarCache.set(key, gravatar);
 
     return gravatar;