Tab not expanded to spaces while writing markdown

[Syphdias]

Describe the bug
When in markdown, using the tab key results in an actual tab instead of expanding to two spaces.

To Reproduce
Steps to reproduce the behavior:

1. Go to the edit page of a markdown site
2. type the following (replace <tab> with pressing tab):

- correct (level 1 color)
  - this is how it should work (level 2 color)
  - this even gets nicely indented (level 2 color)
- wrong (level 1 color)
<tab>- this has the wrong color (level 1 color)
  - this is the correct color (level 2) and automatically indented with two spaces

Expected behavior
tab gets expanded to two spaces (which should result in correct syntax highlighting)

Screenshots

With text highlighted so the tab is more obvious:

Host Info (please complete the following information):

• OS: Docker
• Wiki.js version: 2.0.12
• Database engine: PostgreSQL 11.6