Remove jquery contextmenu

What does this MR do?

Removes jQuery contextmenu plugin and usage and refactors away some view code duplication into a helper function

How confident are you it won't break things if deployed?

confident (for the use cases I'm aware of)

Links to related issues

• &22 (closed)

How to test

1. Join stores
2. Apply for stores
3. Manage stores
4. Wait at stores
5. Check dashboard

Screenshots (if applicable)

no design / frontend change except for removing the barely-used context menu (no functionality is removed there however)

Checklist

• no test, as I'm basically just removing things
• no unrelated changes
• asked someone for a code review
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.