Bump tsconfig target from ES2016 to ES2017

[LMNTL]

Checklist

1. If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. Ensure the tests pass
4. Make sure that your code lints and that you've followed our coding style
5. Write a title and, if necessary, a description of your work suitable for publishing in our release notes
6. Mention any related issues in this repository (as #ISSUE) and in other repositories (as kobotoolbox/other#ISSUE)
7. Open an issue in the docs if there are UI/UX changes

Description

Change TypeScript compilation target from ES2016 to ES2017.

Notes

This is intended to make TypeScript recognize Object.entries() and Object.values(). See kpi#4660 for an example of hacking the issue (using as keyof {Type}). ES2017 is at ~93% use in the wild based on caniuse.com stats, which satisfies our availability requirements.

This should (🙏🏽) be fully forward compatible. TypeScript doesn't complain at compile time and I haven't been able to make it spit any new errors at me from the SPA. But please do give it some testing.