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/test/java/org/whispersystems/textsecuregcm/util/NicknameValidatorTest.java

@@ -12,14 +12,14 @@ import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
 
-class UsernameValidatorTest {
+class NicknameValidatorTest {
 
   @ParameterizedTest
   @MethodSource
   void isValid(final String username, final boolean expectValid) {
-    final UsernameValidator usernameValidator = new UsernameValidator();
+    final NicknameValidator nicknameValidator = new NicknameValidator();
 
-    assertEquals(expectValid, usernameValidator.isValid(username, null));
+    assertEquals(expectValid, nicknameValidator.isValid(username, null));
   }
 
   private static Stream<Arguments> isValid() {
@@ -28,8 +28,8 @@ class UsernameValidatorTest {
         Arguments.of("_test", true),
         Arguments.of("test123", true),
         Arguments.of("a", false), // Too short
-        Arguments.of("thisIsAReallyReallyReallyLongUsernameThatWeWouldNotAllow", false),
-        Arguments.of("Illegal character", false),
+        Arguments.of("thisisareallyreallyreallylongusernamethatwewouldnotalllow", false),
+        Arguments.of("illegal character", false),
         Arguments.of("0test", false), // Illegal first character
         Arguments.of("pаypal", false), // Unicode confusable characters
         Arguments.of("test\uD83D\uDC4E", false), // Emoji
@@ -38,19 +38,4 @@ class UsernameValidatorTest {
         Arguments.of(null, false)
     );
   }
-
-  @ParameterizedTest
-  @MethodSource
-  void getCanonicalUsername(final String username, final String expectedCanonicalUsername) {
-    assertEquals(expectedCanonicalUsername, UsernameValidator.getCanonicalUsername(username));
-  }
-
-  private static Stream<Arguments> getCanonicalUsername() {
-    return Stream.of(
-        Arguments.of("test", "test"),
-        Arguments.of("TEst", "test"),
-        Arguments.of("t_e_S_T", "t_e_s_t"),
-        Arguments.of(null, null)
-    );
-  }
 }