Add ember-appuniversum

[Dietr]

Add ember-appuniversum as dependency so we can remove the duplicate components in plugins over time.

• simplified the theming: bundled all variables in one _s-theme file
• seperated the custom components in a file so we can only load these in Gelinkt Notuleren
• added some ember-appuniversum variables so we maintain the say style (purple primary color)

Next steps:

• update Gelinkt Notuleren. See also: Feature/use appuniversum in editor frontend-gelinkt-notuleren#212
• refactor all plugins to use ember-appuniversum so we can remove the deprecated components
• convert appuniversum sass variables to css variables to remove the sass dependency for vendors

Open questions:

• how do we best handle different versions of ember-appuniversum in GN and ember-rdfa-editor / plugins?

[nvdk]

On the open questions:

• plugins should specify app universum as a peerdependency and be as lenient as possible on version requirements.

[nvdk]

@Dietr I'm guessing we will also need to update https://github.com/lblod/frontend-embeddable-notule-editor

[nvdk]

looks like the strong tag doens't look bold anymore in this branch. other than that I found no immediate issues.

[abeforgit]

On the open questions:

* plugins should specify app universum as a peerdependency and be as lenient as possible on version requirements.

Shouldn't the editor itself then also specify appuniversum as a peerdep?