fix(eslint-plugin): [no-invalid-this] allow "this" in class property definitions

[krisztianb]

Fixes #491

This should fix the things mentioned in the issue. I also added some tests for the new cases.
It seemed almost too easy to fix, so please let me know if I'm missing anything.

Thanks.

[typescript-eslint]

Thanks for the PR, @krisztianb!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

---

🙏 Please, if you or your company is finding typescript-eslint valuable, help us sustain the project by sponsoring it transparently on https://opencollective.com/typescript-eslint. As a thank you, your profile/company logo will be added to our main README which receives thousands of unique visitors per day.

[codecov]

Codecov Report

Merging #2685 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2685   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files         294      294           
  Lines        9676     9678    +2     
  Branches     2713     2713           
=======================================
+ Hits         8978     8980    +2     
  Misses        330      330           
  Partials      368      368           

Flag	Coverage Δ
#unittest	92.78% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ackages/eslint-plugin/src/rules/no-invalid-this.ts	95.83% <100.00%> (+0.37%)	⬆️

[bradzacher]

LGTM - thanks for fixing this!