Clean up imports by using path aliases and barrel files

[rgoj]

Our imports are currently barely readable and difficult to write:

Using path aliases (especially for SharedModule!) and barrel files will make things much nicer. This can probably be done gradually.

Some resources:

• https://levelup.gitconnected.com/shorter-import-paths-in-angular-8141734090e4
• https://www.angularnation.net/posts/angular-architect-training-july-2021-week-1-session-style-and-structure-core-principles

[rgoj]

@maptio-shared is already set up (not barrel file, but at least the path alias), so this could be repeated elsewhere too.

[rgoj]

Going through some of the code and updating some of these might be a nice task to get started with the codebase, so adding the "good first issue" tag. Perhaps not a satisfying change from a user perspective, but certainly so from a clean code perspective :) Any PRs updating even a small number of files - to make some progress on this - welcome.

I'm not sure yet about using barrel files, haven't done that anywhere in the code, but certainly path aliases are there already in many places, so it'd be nice to spread that throughout the app.