Make sure there are options to set the extension ID and an enabled flag for
both Chrome and Firefox.

Removed checking for deprecated configuration options: desktopSharing,
chromeExtensionId and minChromeExtVersion.

Lib-jitsi-meet uses Temasys on Internet Explorer and Safari so it does
not make sense to require its adapter.screenshare.js on React Native,
for example, where it fails anyway.

I added reporting of Promise's reject reason in b10a1fc while merging
jitsi/ss_postinstall_fix since Promise's reject function has such an
argument. Hristo Terezov disagreed with it because reason is undefined
in the use case and found it to be confusing so I'm reverting to what he
intended in the first place.

Simplify the source code by using some ES6 features such as arrow
functions to avoid the use of 'var self = this', the shorthand for
method definitions in object initializers.

Our JSHint configuration is not extensive and we have excluded multiple
files from linting. Additionally, we have seen JSHint to be unable to
parse newer ECMAScript features such as the object spread operator
(proposal).

On the other hand, we have seen ESLint to beautifully work on React and
React Native source code in addition to ES2015 and later. Introduce
ESLint alongside JSHint as an intermediate step to eventually switching
from JSHint to ESLint.

As our source code does not fully follow even the JSHint rules we have,
it is very difficult to introduce ESLint with many rules. At the
beginning, introduce it with as little rules as possible. Morever,
ESLint is able to automatically fix certain rule offenders so once we
have ESLint merged we can incrementally add new rules while keeping our
commits' modification on as little subjects as possible.