Remove unused frontend code

[devmount]

Describe the bug
Throughout the years and several redesigns, the frontend codebase has a few functions, Vue-provide statements or exports, that are defined but not used anymore and should be removed.

Findings
The following (wip) list contains potentially removable code snippets.

Note

The lines listed are only the definition. The actual number of lines that can be deleted is bigger in most cases.

• appointment/frontend/src/utils.ts

  Line 104 in 6965d85

  export const initials = (name: string): string => {

• appointment/frontend/src/keys.ts

  Line 22 in 6965d85

  export const isoFirstDayOfWeekKey = Symbol('isoFirstDayOfWeek') as InjectionKey<number>;

• appointment/frontend/src/keys.ts

  Line 26 in 6965d85

  export const bookingUrlKey = Symbol('bookingUrl') as InjectionKey<string>;

• appointment/frontend/src/definitions.ts

  Line 39 in 6965d85

  export enum SubscriberLevels {

• appointment/frontend/src/definitions.ts

  Line 84 in 6965d85

  export enum BookingsViews {

• appointment/frontend/src/definitions.ts

  Line 94 in 6965d85

  export enum BookingsTableColumns {

• appointment/frontend/src/definitions.ts

  Line 105 in 6965d85

  export enum BookingsTableFilterOptions {

• appointment/frontend/src/definitions.ts

  Line 118 in 6965d85

  export enum BookingsViewTypes {

• appointment/frontend/src/definitions.ts

  Line 182 in 6965d85

  export enum ModalStates {

• appointment/frontend/src/definitions.ts

  Line 250 in 6965d85

  export enum ColorPalette {

• appointment/frontend/src/models.ts

  Line 377 in 6965d85

  export type CalendarResponse = UseFetchReturn<Calendar|Exception>;

• appointment/frontend/src/models.ts

  Line 388 in 6965d85

  export type StringListResponse = UseFetchReturn<string[]>;

Additional context
I found those when implementing another feature. So we can just keep an eye open for dead code and throw possible findings in here until we purge the list.