GitLab CI - ESLint - no-var and prefer-const

Fixing "var" statements, and changing "let" to "const" where not modified, reduces the number of errors considerably.

✖ 247 problems (204 errors, 43 warnings)

Thanks for your work on this @lostcarpark.

While testing these changes I did also find a regression in the end date not hiding, which looks to go back to the move to strict comparisons in a previous issue. I incorporated that fix here as well.

Merged in.

Automatically closed - issue fixed for 2 weeks with no activity.