Change icons for login and join buttons

What does this MR do?

Changes the icon for the login and join buttons on topbar, public profile and loggedInFixedNav.

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

100%

Links to related issues

https://foodsharing.freshdesk.com/a/tickets/1009

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Check the icon of the login button
3. Open a profile without logged in
4. Check the icon of the login button on the public profile
5. Log in as foodsharer (user1)
6. Check the icon on topbar

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• 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.

Release notes text

Das Login-Logo wurde geändert, um den Unterschied zu dem "Mach mit"-Button zu verdeutlichen.