[Admin] Fix Square Logos appearance

[michaelmichael]

Description
Ref #3692

This just adds an explicit height to the .brand-link div in admin. This height is controlled by the $main-header-height variable. Then it sets the image to have a max-width/height of 100%. It should prevent logos of any size or aspect ratio from overflowing and preventing access to the ui. See square, tall, and wide logos below:

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[jarednorman]

One nit, but otherwise I'm fine with this solution. Could probably be done slightly more cleanly with flexbox if you wanted to take a stab at that, though.

[jarednorman]

Looks good to me, but do you mind squashing the two commits together for a clean git history?

[michaelmichael]

Ok, squashed into a single commit.

[aldesantis]

Thanks @michaelmichael!