fixed linter warnings

frontend/.eslintrc.js

frontend/src/App.vue

frontend/src/api.ts

frontend/src/components/AutoLogout.vue

frontend/src/components/LabelStatistics.vue

frontend/src/components/PasswordChangeDialog.vue

frontend/src/components/feedback_labels/FeedbackLabel.vue

...tend/src/components/feedback_labels/FeedbackLabelForm.vue

...d/src/components/feedback_labels/FeedbackLabelUpdater.vue

...end/src/components/feedback_labels/FeedbackLabelsList.vue

frontend/src/components/feedback_labels/LabelSelector.vue

...end/src/components/feedback_list/FeedbackListHelpCard.vue

...nd/src/components/feedback_list/FeedbackSearchOptions.vue

frontend/src/components/feedback_list/FeedbackTable.vue

frontend/src/components/mixins/commentLabelSelector.ts

frontend/src/components/mixins/exportMixin.ts

frontend/src/components/student_list/StudentList.vue

.../src/components/submission_notes/SubmissionCorrection.vue

...tend/src/components/submission_notes/base/CommentForm.vue

.../src/components/submission_notes/base/FeedbackComment.vue