created keyboard shortcuts

[maansi-nema]

What does this PR do?

This PR helps to solve the issue on the task development board of implementing key board shortcuts for users to have an easier time writing messages. We implemented the following shortcuts:
Strg + B = bold
Strg + I = italic
Strg + U = underline
Alt + Shift + 5 or Strg + Alt + 5 = strikethrough

Links to related issues

https://community.karrot.world/t/introduce-shortcuts-for-text-formatting/1316

Checklist

• [Y] added a test, or explain why one is not needed/possible...
• [Y] no unrelated changes
• [Y] asked someone for a code review
• [Y] joined #karrot:matrix.org
• [N] added an entry to CHANGELOG.md (description, pull request link, username(s))
• [Y] tried out on a mobile device (does everything work as expected on a smaller screen?)

[nicksellen]

The topic you selected to work on was assigned to Nathalie:

Although it was inactive for some time, in these cases it would be a good practise to check first before starting work. Originally, I understand the task to be a good learning exercise for Nathalie learning to code.

Unfortunately we have little capacity for supporting ungrad students, as explained over at https://codeberg.org/karrot/karrot-frontend/src/branch/main/CONTRIBUTE.md

I did checkout your branch, but it does not load, and there is this error in the console:

You also likely need to rebase your changes on top of the latest main branch, which may or may not resolve that issue, there was a recent switch of the emoji library to a maintained branch, which may or may not be related to the error.

Additionally, we have switched to codeberg so pull requests should be created over there https://codeberg.org/karrot/karrot-frontend not in GitHub, that is probably not clear right now, as we only recently moved.

I think you are a student at university of Michigan too right? Would you be able to give me the contact email address of somebody there? I would like to update them on the situation, as we cannot provide the support needed by students.

Thanks, and sorry for the not-too-positive message here.