Refactor: Replace username validation with schema from Tildes source code.

This replaces the simple >3 check with the correct min and max lengths and
Regex that are in the Tildes source code.

source/ts/scripts/user-labels.ts

@@ -192,6 +192,7 @@ function addLabelsToUsernames(settings: Settings): void {
     const username: string = element
       .textContent!.replace(/@/g, '')
       .toLowerCase();
+
     const addLabelSpan: HTMLSpanElement = createElementFromString(
       `<span class="trx-user-label-add" data-trx-username="${username}">[+]</span>`
     );

source/ts/scripts/user-labels/label-form.ts

@@ -5,7 +5,8 @@ import {
   log,
   isColorBright,
   getCurrentThemeKey,
-  querySelector
+  querySelector,
+  isValidTildesUsername
 } from '../../utilities';
 import {themeColors, ThemeKey} from '../../theme-colors';
 
@@ -95,8 +96,8 @@ export function getLabelFormValues(): Except<UserLabel, 'id'> | undefined {
     username: usernameInput.value.toLowerCase()
   };
 
-  if (data.username.length < 3) {
-    log('No username was provided to add a label to.', true);
+  if (!isValidTildesUsername(data.username)) {
+    log(`Invalid Tildes username detected: ${data.username}`);
     usernameInput.classList.add('trx-invalid');
     return undefined;
   }

source/ts/utilities.ts

@@ -252,3 +252,13 @@ export function flashMessage(message: string, error = false): void {
     }
   }, 5000);
 }
+
+// Validation copied from Tildes source code:
+// https://gitlab.com/tildes/tildes/blob/master/tildes/tildes/schemas/user.py
+export function isValidTildesUsername(username: string): boolean {
+  return (
+    username.length > 3 &&
+    username.length < 20 &&
+    /^[a-z0-9]([a-z0-9]|[_-](?![_-]))*[a-z0-9]$/i.exec(username) !== null
+  );
+}