Refactor/eslint

[Gijsdeman]

Description

Moves eslint to @gewis/eslint-config-typescript. This introduces quite some issues, as this config uses type information for more accurate code.

Since there are so many changes to be made, it is probably best to work through this file by file, getting rid of all the errors in separate commits and squash this later.

Types of changes

• Refactoring (Changes that neither fixes a bug nor adds a feature)

[Gijsdeman]

Fixed eslint issues wherever possible. However, some files need a bigger overhaul to resolve the remaining issues:

• FileService.ts: needs some more generics
• ActivityService.ts: needs some more generics
• rawQueries.ts: needs to be rewritten to use query builders
• activity.ts: has some generics that didn't work out, need to take another look
• filters.ts: not sure if it is still required with new typeorm updates

I will leave these for separate issues, however, and this can be merged to at least move to newer version of eslint.