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All: Update ESLint to v7

Update from ESLint v4 to v7. Mainly motivated by the improvement
to the `ignorePatterns` capability, which I plan to use in the
next commit.

New issues uncovered that were fixed:

```
/qunit/reporter/diff.js
  974:15  error  Do not access Object.prototype method 'hasOwnProperty'
                 from target object  no-prototype-builtins

/qunit/reporter/html.js
   42:1  error  Expected indentation of 1 tab but found 0   indent
  136:1  error  Expected indentation of 3 tabs but found 2  indent

/qunit/src/export.js
  9:1  error  Expected indentation of 2 tabs but found 1  indent

/qunit/build/coverage-bridge.js
  1:34  error  'require' is already defined as a built-in global
               variable  no-redeclare
```

Confirmed these are all genuine mistakes indeed.

* eslint is indirectly provided by grunt-eslint.
  grunt-eslint 20 provided eslint 4.
  grunt-eslint 23 provides eslint 7.

* eslint-plugin-html 4 is not compatible with eslint 7,
  and is thus updated as well.

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