refactor: menu consistencies

[alexbarnsley]

It is difficult to position the arrow relating to the menus, so we've decided to remove them. They didn't serve many purposes. The plugins menu list has also been "fixed" to be in dark mode, to match the other 2 menus from the sidebar, and the position has been fixed when there is only 1 plugin menu item.

Summary

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

Merging #1555 into develop will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           develop    #1555   +/-   ##
========================================
  Coverage    45.68%   45.68%           
========================================
  Files          274      274           
  Lines         6849     6849           
  Branches      1342     1342           
========================================
  Hits          3129     3129           
  Misses        3558     3558           
  Partials       162      162

Impacted Files	Coverage Δ
...nderer/components/Menu/MenuOptions/MenuOptions.vue	100% <ø> (ø)	⬆️
.../components/App/AppSidemenu/AppSidemenuPlugins.vue	0% <0%> (ø)	⬆️

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