refactor(site): generalize UserCell component (#484)

Summary:

This is a first step in porting over v1 AuditLog in a refactored/cleaned
up fashion. The existing `UserCell` component was generalized for re-use
across various tables (AuditLog, Users, Orgs).

Details:

- Move UserCell to `components/Table/Cells`
- Add tests and stories for UserCell


Impact:

This unblocks future work in list views like the audit log, user management
panel and organizations management panel.

Relations:

- This commit relates to https://github.com/coder/coder/issues/472, but does not finish it.
- This commit should not merge until after https://github.com/coder/coder/pull/465 and https://github.com/coder/coder/pull/483 because it's
based on them.

site/src/util/first-letter.test.ts

@@ -0,0 +1,11 @@
+import { firstLetter } from "./first-letter"
+
+describe("first-letter", () => {
+  it.each<[string, string]>([
+    ["", ""],
+    ["User", "U"],
+    ["test", "T"],
+  ])(`firstLetter(%p) returns %p`, (input, expected) => {
+    expect(firstLetter(input)).toBe(expected)
+  })
+})

site/src/util/first-letter.ts

@@ -0,0 +1,10 @@
+/**
+ * firstLetter extracts the first character and returns it, uppercased.
+ */
+export const firstLetter = (str: string): string => {
+  if (str.length > 0) {
+    return str[0].toLocaleUpperCase()
+  }
+
+  return ""
+}