Add audience and event relations to SplashEvent

[oleast]

What kind of a pull request is this?

• QA / Code Review

Code Checklist

• The code follows dotkom code style
• The code passes the defined tests
• I have added tests for the code I added
• I have provided documentation for the code I added
• The code is ready to be merged

(Possible) Breaking Changes

• The changes are backwards compatible

will need some manual renaming of splash events as they have hard-coded audience groups e.g. [Bachelor].

Description of changes

• Add the option to add a target audience for a splash-event, e.g. [Bachelor] or [Master].
• Adds the option to relate a splash event to a regular event, mostly for linking purposes.

Resolves #2291

Screenshot(s) if appropriate