Add support for generating discriminators

- adds `PUT accounts/username` endpoint
- adds `GET accounts/username/{username}` to lookup aci by username
- deletes `PUT accounts/username/{username}`, `GET profile/username/{username}`
- adds randomized discriminator generation

service/src/main/java/org/whispersystems/textsecuregcm/configuration/UsernameConfiguration.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2013-2020 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.configuration;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import javax.validation.constraints.Min;
+
+public class UsernameConfiguration {
+
+  @JsonProperty
+  @Min(1)
+  private int discriminatorInitialWidth = 4;
+
+  @JsonProperty
+  @Min(1)
+  private int discriminatorMaxWidth = 9;
+
+  @JsonProperty
+  @Min(1)
+  private int attemptsPerWidth = 10;
+
+  public int getDiscriminatorInitialWidth() {
+    return discriminatorInitialWidth;
+  }
+
+  public int getDiscriminatorMaxWidth() {
+    return discriminatorMaxWidth;
+  }
+
+  public int getAttemptsPerWidth() {
+    return attemptsPerWidth;
+  }
+}