Implement alphabetical view in sidebar using Vue

[UnniKohonen]

Description of the enhancement

Adding a Vue component for alphabetical index in the sidebar.

What new functionalities would the enhancement make possible?

Most functionality in alphabetical index is complete but some features are still missing (more can be added as they come up):

• 1. CSS
  • a. Highlighting currently open concept (removing #tab-hierarchy from this line should fix it)
  • b. Correct sidebar height
  • c. Scrollbar visibility (in some browsers, scrollbar is shown in the middle of the sidebar before the component is loaded fully)
  • d. Resizing browser window breaks width
• 2. Loading new concepts as list is scrolled
• 3. Displaying concepts and index letters in content language
• 4. Show full alphabetical index (configuration property fullAlphabeticalIndex)