Add eslint to ensure we are compatible with Wikimedia standards

[bawolff]

Per https://en.wikipedia.org/wiki/MediaWiki_talk:Gadget-owidslider.js - we had an issue where we used js features not supported on safari. We are supposed to follow https://www.mediawiki.org/wiki/Compatibility#Browsers

Setting up linting with eslint-config-wikimedia with grunt so that we can easily test by running npm test from the command line would help us ensure that we don't repeat this issue in the future.

[lgies]

REL1_43 branch uses
"eslint-config-wikimedia": "^0.28.2"

[lgies]

Adding config. Fixing errors.
Current status:

Count	Error Type
255	no-mixed-spaces-and-tabs
122	no-undef
37	block-scoped-var
26	es-x/no-block-scoped-variables
21	eqeqeq
14	no-restricted-properties
14	max-len
13	no-console
11	no-unused-vars
11	no-shadow
10	no-implicit-globals
6	camelcase
5	no-tabs
4	es-x/no-array-prototype-includes
3	no-redeclare
2	es-x/no-string-prototype-includes
2	es-x/no-object-entries
2	es-x/no-destructuring
1	no-useless-concat
1	new-cap
1	jsdoc/require-param-type
1	jsdoc/require-asterisk-prefix
1	es-x/no-string-prototype-replaceall
1	es-x/no-object-values
1	es-x/no-object-assign
1	es-x/no-for-of-loops

[lgies]

type	count
indent	239
no-undef	35
es-x/no-block-scoped-variables	26
block-scoped-var	24
max-len	15
no-console	13
no-shadow	11
no-implicit-globals	10
es-x/no-array-prototype-includes	4
no-redeclare	4
es-x/no-destructuring	2
es-x/no-object-entries	2
es-x/no-string-prototype-includes	2
no-unused-vars	2
es-x/no-for-of-loops	1
es-x/no-object-assign	1
es-x/no-object-values	1
es-x/no-string-prototype-replaceall	1