chore: add "aria-label" and "role" attributes to AddCommentFormView component

This change is also needed by #1484:
- the "role" attribute makes the alert "searchable" by screen.getByRole()
- the "aria-label" attribute makes the form "searchable" by screen.getByRole()
  (testing-library/dom-testing-library#474 (comment))

Part of #1493

Author: php-coder

Diff for: src/main/frontend/src/components/AddCommentForm.js

@@ -93,10 +93,13 @@ class AddCommentFormView extends React.PureComponent {
 		const hasValidationErrors = validationErrors.length > 0;
 		return (
 			<div className="col-sm-12 form-group">
-				<form id="add-comment-form" className={ `form-horizontal ${hasValidationErrors ? 'has-error' : ''}` }
+				<form id="add-comment-form"
+					aria-label={ l10n['t_add_comment'] || 'Add a comment' }
+					className={ `form-horizontal ${hasValidationErrors ? 'has-error' : ''}` }
 					onSubmit={ handleSubmit }>
 					<div
 						id="add-comment-failed-msg"
+						role="alert"
 						className={ `alert alert-danger text-center col-sm-8 col-sm-offset-2 ${hasServerError ? '' : 'hidden'}` }>
 						{ l10n['t_server_error'] || 'Server error' }
 					</div>

Diff for: src/main/webapp/WEB-INF/views/series/info.html

@@ -1251,6 +1251,7 @@ <h5 th:text="#{t_add_info_who_selling_series}">Add info about selling/buying thi
 				'csrfTokenValue': [[ ${_csrf.token} ]],
 				'l10n': {
 					't_server_error': [[ #{t_server_error} ]],
+					't_add_comment':  [[ #{t_add_comment} ]],
 					't_comment':      [[ #{t_comment} ]],
 					't_add':          [[ #{t_add} ]]
 				}