Use LoginMenu from multinet-components

[JackWilb]

Does this PR close any open issues?

No

Give a longer description of what this PR addresses and why it's needed

This is the first step on the path to using reusable components from a central multinet-components repo. I have preliminarily moved the LoginMenu component there, and I'm using that implementation here.

To make this work, I had to change the implementation of the mdi-icons. We now use the same strategy as the other client apps, where we just get a css link in the index.html file.

Provide pictures/videos of the behavior before and after these changes (optional)

Before:

After:

Are there any additional TODOs before this PR is ready to go?

TODOs:

• [x] Fix MDI icons for avatar
• [x] Delete old MDI icon update branch, authored by @AlmightyYakob

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