Add dropdown for SteamUserPermissions config prop

[MrBurrBurr]

Closes #1541

@Abrynos Feel free to check it out and let me know what you think.

[Abrynos]

The dropdown is misaligned. As you can see, the grey background of the popup starts higher than the bottom of the dropdown next to it (which is properly aligned). Apart from that, the "arrow" for the dropdown is a different one from all the other dropdown menus.

Functionally it does everything I imagined. Good job!

[Abrynos]

The mouse over effect is still different (colored text on the bot dropdown, white text for the permission dropdown).

For the bot, the dropdown menu both has a horizontal scrollbar as well as it resizes itself.

When opening the permission enumeration dropdown with a click while the bot-dropdown is open, it assumes the same width and thus is bigger than it should be.

For bots not logged in, the name in the parentheses will contain the text null.

[MrBurrBurr]

The mouse over effect is still different (colored text on the bot dropdown, white text for the permission dropdown).

For the bot, the dropdown menu both has a horizontal scrollbar as well as it resizes itself.

When opening the permission enumeration dropdown with a click while the bot-dropdown is open, it assumes the same width and thus is bigger than it should be.

I re-used vue-multiselect for the bot dropdown. It's the same as we use for the Mass Editor.

For the permissions dropdown we use the browser's default styling.

This is why they look and behave differently. We should align the styling at some point in the future but I do not think this PR is the right place for it. It should have it's own PR.

The dropdown is misaligned. As you can see, the grey background of the popup starts higher than the bottom of the dropdown next to it (which is properly aligned).

This on the other hand should be fixed with this PR.

For bots not logged in, the name in the parentheses will contain the text null.

Good catch, i fixed it.